[gazelle] Add support for resources in gradle sourcesets

[shs96c]

This means that if we see a directory structure like:

src/foo/java
src/foo/resources

We'll now generate a target in the resources directory which will be included in the target generated in the java directory. This models the behaviour of both maven and gradle which will put those resources into the single jar that they build.

[stevebarrau]

Looks good. Thanks for having a look.

Maybe remove some of the AI handholding intra-function comments.

I was thinking about the case where the resources are used by compiler plugins and should thus be deps instead of runtimeDeps, but we can have a keep directive for this (I have no example of this but I am pretty sure some plugin in the wild does this).

[H5-O5]

@shs96c This doesn't work for me. Could you explain why it works?

https://github.com/shs96c/rules_jvm/blob/2850f1a8019cb8bd59100c699114de7e3624da1d/java/gazelle/testdata/resources_custom_sourceset/src/sample/resources/BUILD.out

pkg_files(
    name = "resources",
    srcs = [
        "com/example/app.properties",
        "config.xml",
    ],
    strip_prefix = "src/sample/resources",
)

Basically if I just bazel build //path/to/dir:resources I got Error in fail: Could not strip prefix 'src/sample/resources'