RFR: 8304367: jlink --include-locales=* attempts to parse non .class resource files with classfile reader

[Naoto Sato]

This is a blocker for [JDK-8294972](https://bugs.openjdk.org/browse/JDK-8294972). The fix is to weed out non-class files for calling `ClassReader` by checking the file extension. Regression tests are not provided as it is not possible to detect the fix from the artifact jimage, but manually confirmed it would effectively weed out `LineBreakIteratorData_th`, `WordBreakIteratorData_th`, and `thai_dict` files by running `IncludeLocalesPluginTest.java`.

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Commit messages:
 - 8304367: jlink --include-locales=* attempts to parse non .class resource files with classfile reader

Changes: https://git.openjdk.org/jdk/pull/13067/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13067&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8304367
  Stats: 3 lines in 1 file changed: 1 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/13067.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/13067/head:pull/13067

PR: https://git.openjdk.org/jdk/pull/13067