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## Summary
Minimal set of changes to ship a `module-info.java`, and support JVM 1.8 for the `annotations` module only.
### Reasoning
It costs almost nothing to support _only_ the annotations on JVM 1.8, for projects which transitively include `error-prone-annotations`, which can happen via a simple dependency on something like Guava. But, it would be ideal to [ship a `module-info.java`](#2649), so `annotations` is now a `Multi-Release` JAR, with a `module-info.class` in `META-INF/versions/9`.
I am working downstream to [support JPMS in Guava](google/guava#2970), and this PR will be necessary for that to eventually land (without hackery). [Checker Framework recently merged their support](typetools/checker-framework#6326) which means Error Prone is one of the last things on the classpath for Guava without a `module-info.java` definition.
Automatic Modules also aren't the answer because they cannot be used with `jlink`. Such dependencies must be processed by tools like Moditect before they can be used in fully modular Java builds. Because Error Prone Annotations is a dependency in Guava, and is itself very popular, many projects need Error Prone to adopt JPMS so they can ship their own code with modular Java support.
There may be libraries out there that depend on the annotations _and not the compiler_, who wish to ship a MRJAR and retain JVM 1.8 support (Guava).
### Non-release version number change
One wrinkle here is that the Maven project version [is used unconditionally](https://github.com/apache/maven-compiler-plugin/blob/74cfc72acae4f55708bca189b2170167e83df6b3/src/main/java/org/apache/maven/plugin/compiler/CompilerMojo.java#L304-L305) [as the `--module-version`][0]; however, [`HEAD-SNAPSHOT` is not a valid module identifier](https://lists.apache.org/thread/qtghq13qgjoc4pn6ovsdxgnjnm4ozv4d). This leaves only a few choices to support JPMS through recent (`3.8.x+`) Maven Compiler plugin versions:
- `HEAD-SNAPSHOT` needs to become `1.0-HEAD-SNAPSHOT`, and then it is a valid `--module-version`
- Or, Maven Compiler Plugin needs to ship a bugfix and Error Prone will need to wait for a release (if a bugfix ships at all)
I understand this can be a breaking change somewhere within Google, but I don't see a way around this without resorting to severe build hacks. I've gotten it to build anyway by building the `module-info.java` only in a separate Maven project, and then copying it into the JAR at the last moment, but there are serious issues with that route so I suggest it be abandoned in favor of a version string change during dev, if possible.
## Changelog
- feat: add `module-info` to `annotations` module
- feat: ship `annotations` as a `Multi-Release` JAR
- feat: support `1.8` through latest JDK for `annotations` module
- fix: remove automatic module definition from `annotations` module
- fix: `HEAD-SNAPSHOT` → `1.0-HEAD-SNAPSHOT`, because of a Maven Compiler Plugin issue [precluding use as a module version][0]
Fixes and closes#2649
[0]: https://issues.apache.org/jira/browse/MCOMPILER-579Fixes#4311
FUTURE_COPYBARA_INTEGRATE_REVIEW=#4311 from sgammon:feat/jpms d209b0c
PiperOrigin-RevId: 614005681
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