Fix JavaLocator, close #619

[slandelle]

Motivation:

1. We should prefer the Java installation used to run maven to what's defined in JAVA_HOME
2. We should prefer JDK over JRE (for old versions that used to have one) as we need javac for compiling Java code in mixed projects

Modifications:

Resolve with the following chain:

• prefer toolchain if defined
• try to use "java.home" System prop to resolve a JDK, a JRE otherwise
• fall back to JAVA_HOME env var

Result:

Behavior consistent with maven-compiler-plugin and org.codehaus.plexus.compiler.javac.JavacCompiler#getJavacExecutable.

In particular:

• we no longer unexpectedly jump to a different Java install because JAVA_HOME env var is defined
• we no longer crash on Java 8 when trying to compile mixed projects and JAVA_HOME env var is undefined or doesn't point to a JDK.

[slandelle]

@roti Could you please give this branch a try?

[slandelle]

Personaly, I prefer when there is only one return with result (maybe some into the guardian/pre-check section).

@davidB Honestly, I can't find a way with Java's poor tools for composability. Feel free to refactor if you have some nice ideas.

[roti]

@slandelle It works for me. Thanks.