Clean up checker-qual module-info build configuration and avoid bare 'src/main' Eclipse workaround #2130

Problem Description

In checker-qual/build.gradle, the module_info source set is currently configured as:

sourceSets {
    main {
        java {
            exclude 'module-info.java'
        }
    }
    module_info {
        java {
            srcDirs ('src/main')
        }
    }
}

Setting srcDirs ('src/main') on module_info causes several drawbacks:

1. IDE classpath collision: Gradle's eclipse plugin registers src/main alongside src/main/java as source folders in checker-qual/.classpath. Eclipse and VS Code (Eclipse JDT LS) reject nested source folders without mutual exclusion filters, producing a build error (Cannot nest 'checker-qual/src/main/java' inside 'checker-qual/src/main').
   To unblock IDEs, a workaround was added in root build.gradle (PR Improve VS Code and Eclipse support #1518) filtering out e.path == 'src/main' in eclipse.classpath.file.whenMerged.
2. Duplicate compilation: Because srcDirs ('src/main') covers all source files under src/main, sourceSets.module_info.java contains all 363 Java files in checker-qual. compileJava9 therefore compiles all annotations under --release 9 before compileJava overwrites them with Java 8 bytecode.
3. Task workarounds: Gradle creates auto-generated tasks for module_info (e.g., compileModule_infoJava), requiring workarounds like tasks.named('compileModule_infoJava') { enabled = false }.

Proposed Cleanups

Investigate cleaner alternatives to avoid generating the bare src/main source set and compiling the whole library twice:

• Option 1 (Compile module-info.java directly): Remove sourceSets.module_info entirely. Register compileJava9 as a standalone task with source = files('src/main/java/module-info.java') and classpath pointing to sourceSets.main.output.classesDirs. This keeps module-info.java at src/main/java/module-info.java without moving files or introducing dummy source sets.
• Option 2 (Relocate module-info.java): Move module-info.java to src/module_info/java/module-info.java (the default convention for sourceSets.module_info) or src/main/java9/module-info.java, avoiding nesting with src/main/java. Note trade-off: file divergence when merging from upstream typetools.
• Option 3 (Localize Eclipse filter): If sourceSets.module_info is retained for upstream compatibility, localize the Eclipse entry removal to checker-qual/build.gradle rather than applying it globally in root build.gradle.