Checksum validation: checksum files generated for a signature file result in validation failure

With `--require-checksums`, `validateLocalMavenRepo` task fails if there is an ".asc" file for which checksums were generated:
```
[Artifacts Validation] Error detected while reading file /org/jetbrains/kotlinx/kotlinx-collections-immutable/0.0.0-test/kotlinx-collections-immutable-0.0.0-test-javadoc.jar.asc: There are checksum and/or signature files corresponding to an artifact, but the main artifact file does not exist: /org/jetbrains/kotlinx/kotlinx-collections-immutable/0.0.0-test/kotlinx-collections-immutable-0.0.0-test-javadoc.jar.asc
```

While having checksums for signature files is unnecessary and rather a sign of an issue with publication setup, it should not result in verification failures.

Start with the validateLocalMavenRepo task and the --require-checksums option, then reproduce the failure using a repository containing an .asc file and generated checksums. Done means checksum files for signature files no longer cause validation failure while genuine missing main artifacts are still reported.

Written by the indexing model from the issue text.