Feature Request: subpath derived FileSource in build.zig

[Javyre]

Hey, I'd like to get a FileSource relative to another FileSource representing a directory in build.zig.

Example Use-Case:
I am unzipping some binaries and want to then install/link some subdirectories/files from the unzip.
Im trying to avoid implementing a custom Step and just use the builtin ones in a Makeish way.

    ...
    const fetch_skia_zip = b.addSystemCommand(&.{ "curl", "-L", "-o" });
    const skia_zip = fetch_skia_zip.addOutputFileArg("skia-bin.zip");
    fetch_skia_zip.addArg(skia_zip_url);

    const unzip_skia_zip = b.addSystemCommand(&.{"unzip"});
    unzip_skia_zip.step.dependOn(skia_zip);
    unzip_skia_zip.addFileSourceArg(skia_zip);
    unzip_skia_zip.addArg("-d");
    const skia_dir = unzip_skia_zip.addOutputFileArg("skia-bin");

    // want to:
    // install skia_dir/out/.../libskia.a and others
    // link above libskia.a with my zig project
    // add the skia_dir/include directory to the include for my zig project
    
    // would be awesome:
    const skia_include_dir = skia_dir.subPath("include"); // This produces a new FileSource relative to skia_dir.
    ...