[BUG] [openapi-generator-cli] BATCH mode does not recognize multiple or nested '!include' directives.

Bug Report Checklist

[:white_check_mark:] Have you provided a full/minimal spec to reproduce the issue?
[:white_check_mark:] Have you validated the input using an OpenAPI validator (example)?
[:white_check_mark: ] What's the version of OpenAPI Generator used?
[:white_check_mark:] Have you search for related issues/PRs?
[:white_check_mark:] What's the actual output vs expected output?
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Description

openapi-generator-cli BATCH mode does not recognize multiple or nested '!include' directives.

openapi-generator version

4.3.1

OpenAPI declaration file content or url

InputSpec is your petstore-v3.0.yaml

Generation Details

NESTED FAILURE EXAMPLE

# java-common.yaml
generatorName: java
groupId: org.thisisatest.digital
configOptions:
  dateLibrary: java8

# java-jersey2.yaml
'!include': 'java-common.yaml'
library: jersey2

# test-config.yaml
'!include': 'java-jersey2.yaml'
inputSpec: petstore-v3.0.yaml
outputDir: generated/test

Command line: java -jar openapi-generator-cli-4.3.1.jar batch --verbose test_config.yaml

output:

[main] INFO  o.o.codegen.cmd.GenerateBatch - Batch generation using up to 16 threads.
Includes: /Users/fred/openapi-generator-cli/flat
Root: /Users/fred/openapi-generator-cli/flat
[pool-1-thread-1] Generation failed: (NullPointerException) language/generatorName must be specified
java.lang.NullPointerException: language/generatorName must be specified
    at org.apache.commons.lang3.Validate.notEmpty(Validate.java:395)
    at org.openapitools.codegen.config.CodegenConfigurator.toContext(CodegenConfigurator.java:423)
    at org.openapitools.codegen.config.CodegenConfigurator.toClientOptInput(CodegenConfigurator.java:507)
    at org.openapitools.codegen.cmd.GenerateBatch$GenerationRunner.run(GenerateBatch.java:189)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
    at java.lang.Thread.run(Thread.java:748)
COMPLETE.
mbp-fred:flat fred$

SEQUENTIAL FAILURE EXAMPLE

# java-common.yaml
generatorName: java
groupId: org.thisisatest.digital
configOptions:
  dateLibrary: java8

# java-jersey2.yaml
#### REMOVED NESTED INCLUDE
library: jersey2

# test-config.yaml
#### MOVED IT HERE
'!include': 'java-common.yaml'
'!include': 'java-jersey2.yaml'
inputSpec: petstore-v3.0.yaml
outputDir: generated/test

Command line: java -jar openapi-generator-cli-4.3.1.jar batch --verbose test_config.yaml

java -jar openapi-generator-cli-4.3.1.jar batch --verbose test_config.yaml
[main] INFO  o.o.codegen.cmd.GenerateBatch - Batch generation using up to 16 threads.
Includes: /Users/fred/openapi-generator-cli/flat
Root: /Users/fred/openapi-generator-cli/flat
[pool-1-thread-1] Generation failed: (NullPointerException) **language/generatorName must be specified**
java.lang.NullPointerException: language/generatorName must be specified
    at org.apache.commons.lang3.Validate.notEmpty(Validate.java:395)
    at org.openapitools.codegen.config.CodegenConfigurator.toContext(CodegenConfigurator.java:423)
    at org.openapitools.codegen.config.CodegenConfigurator.toClientOptInput(CodegenConfigurator.java:507)
    at org.openapitools.codegen.cmd.GenerateBatch$GenerationRunner.run(GenerateBatch.java:189)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
    at java.lang.Thread.run(Thread.java:748)
COMPLETE.
mbp-fred fred$

REVERSAL OF INCLUDEs WORKS?

If you reverse the two includes in test-config.yaml, the generation seems to work, but the settings in the first included file are ignored.

# java-common.yaml
generatorName: java
groupId: org.thisisatest.digital
configOptions:
  dateLibrary: java8

#java-jersey2.yaml
library: jersey2
groupId: org.thisisatest.injersey2

#test_config.yaml
'!include': 'java-jersey2.yaml'
'!include': 'java-common.yaml'
inputSpec: petstore-v3.0.yaml
outputDir: generated/test

The generation succeeds, but note that the date library is threetenp, and the http/json libraries are the default instead of those specified in java-jersey2.yaml:

dependencies {
    compile 'io.swagger:swagger-annotations:1.5.24'
    compile "com.google.code.findbugs:jsr305:3.0.2"
    compile 'com.squareup.okhttp3:okhttp:3.14.7'
    compile 'com.squareup.okhttp3:logging-interceptor:3.14.7'
    compile 'com.google.code.gson:gson:2.8.6'
    compile 'io.gsonfire:gson-fire:1.8.4'
    compile group: 'org.apache.commons', name: 'commons-lang3', version: '3.10'
    compile 'org.threeten:threetenbp:1.4.3'
    testCompile 'junit:junit:4.13'
}

ACTUAL RESULTS WITHOUT INCLUDES

# test_all_config.yaml
generatorName: java
groupId: org.thisisatest.digital
configOptions:
  dateLibrary: java8
library: jersey2
inputSpec: petstore-v3.0.yaml
outputDir: generated/test

java -jar openapi-generator-cli-4.3.1.jar batch --verbose test_all_config.yaml


dependencies {
    compile "io.swagger:swagger-annotations:$swagger_annotations_version"
    compile "com.google.code.findbugs:jsr305:3.0.2"
    compile "org.glassfish.jersey.core:jersey-client:$jersey_version"
    compile "org.glassfish.jersey.media:jersey-media-multipart:$jersey_version"
    compile "org.glassfish.jersey.media:jersey-media-json-jackson:$jersey_version"
    compile "com.fasterxml.jackson.core:jackson-core:$jackson_version"
    compile "com.fasterxml.jackson.core:jackson-annotations:$jackson_version"
    compile "com.fasterxml.jackson.core:jackson-databind:$jackson_databind_version"
    compile "org.openapitools:jackson-databind-nullable:$jackson_databind_nullable_version"
    compile "com.github.joschi.jackson:jackson-datatype-threetenbp:$threetenbp_version"
    compile "com.brsanthu:migbase64:2.2"
    testCompile "junit:junit:$junit_version"
}
``

##### Steps to reproduce

* Create the yaml files, above. 
* run the given java command.

##### Related issues/PRs

I didn't find any.

##### Suggest a fix

I think the code in [GenerateBatch.java](https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator-cli/src/main/java/org/openapitools/codegen/cmd/GenerateBatch.java), class DynamicSettingsRefSupport, method deserialize doesn't support multiple includes as it uses the TreeNode.get(String) method that can only return a single node.

Also, the code that reads in the included file does not then parse the included data for nested '!include' tokens.

OpenAPITools / openapi-generator