Handle conditionals in Layers definition

[sfirrin]

What does this PR do?

Fixes a bug where the macro would get an error when the Layers definition was not an array, e.g. when it's a CFN conditional. This PR updates the code to handle this situation and add the expected layers to all possible outputs of the conditional

Motivation

Bug

Testing Guidelines

• Added unit tests
• Tested deploying a function with a conditional statement in its Layers definition

Additional Notes

Types of changes

• [x] Bug fix
• [ ] New feature
• [ ] Breaking change
• [ ] Misc (docs, refactoring, dependency upgrade, etc.)

Check all that apply

• [x] This PR's description is comprehensive
• [ ] This PR contains breaking changes that are documented in the description
• [ ] This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• [ ] This PR impacts documentation, and it has been updated (or a ticket has been logged)
• [x] This PR's changes are covered by the automated tests
• [ ] This PR collects user input/sensitive content into Datadog

[codecov-commenter]

Codecov Report

Merging #93 (8ee00d3) into master (e897275) will decrease coverage by 0.94%. The diff coverage is 65.21%.

:exclamation: Current head 8ee00d3 differs from pull request most recent head b0176c5. Consider uploading reports for the commit b0176c5 to get more accurate results

@@            Coverage Diff             @@
##           master      #93      +/-   ##
==========================================
- Coverage   87.39%   86.46%   -0.94%     
==========================================
  Files           8        8              
  Lines         595      613      +18     
  Branches      151      154       +3     
==========================================
+ Hits          520      530      +10     
- Misses         64       72       +8     
  Partials       11       11              

Impacted Files	Coverage Δ
serverless/src/layer.ts	91.34% <63.63%> (-7.51%)	:arrow_down:
serverless/src/index.ts	91.54% <100.00%> (+0.12%)	:arrow_up:

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[sfirrin]

That's a good question @duncanista. AFAIK array and "Fn::If" objects are the only two possible values for Layers in the code, did you have a situation in particular in mind? The CFN doc on the Lambda resource https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-lambda-function.html#cfn-lambda-function-layers says Layers is always a string[] by which I guess it means Layers always has to evaluate to a string[] after being transformed and functions are evaluated. Looking at the doc for intrinsic CFN functions https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/intrinsic-function-reference.html AFAICT the only one that could be an object at the root (rather than an array item) is Fn::If