[MCOMPILER-569] Document implicitly compiled excluded sources

[wilx]

Clarify the compile and testCompile exclusion parameter documentation: an excluded source such as module-info.java may still be compiled when javac finds it implicitly. Link both parameters to <implicit>, which can be set to none to suppress class generation for implicitly referenced sources.

This documents the distinction between the plugin's explicit source filtering and javac's implicit source discovery.

Fixes #778

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[elharo]

I don't understand what "implicitly referenced by the compiler" means.

[wilx]

This is described in https://docs.oracle.com/en/java/javase/17/docs/specs/man/javac.html#searching-for-module-package-and-type-declarations. The relevant lines are

When compiling code for a module, the compiler also needs to have available the declaration of that module.
A successful search may produce a class file, a source file, or both. If both are found, then you can use the -Xprefer option to instruct the compiler which to use.
If a search finds and uses a source file, then by default javac compiles that source file. This behavior can be altered with -implicit.

Feel free to suggest better wording.

[Copilot]

Pull request overview

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Documents that excludes/testExcludes only affect the plugin’s explicit source filtering, and that javac may still compile excluded files when it discovers them implicitly (e.g., module-info.java), pointing users at the implicit setting.

Changes:

• Clarify excludes Javadoc to mention implicit compilation by javac
• Clarify testExcludes Javadoc similarly
• Add a reference to AbstractCompilerMojo#implicit from both parameters

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/main/java/org/apache/maven/plugin/compiler/CompilerMojo.java	Adds Javadoc clarifying that excluded sources can still be compiled implicitly and references implicit.
src/main/java/org/apache/maven/plugin/compiler/TestCompilerMojo.java	Mirrors the same clarification for test compilation excludes and references implicit.

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[wilx]

I have pushed a change with different wording.