Support output group in copy_file

[joca-bt]

Copying a file generated by an output group is not supported, requiring custom rules or workarounds.

Could you consider adding an optional argument output_group to copy_file?

For example, this is our local copy_file implementation:

def _copy_file_impl(name, src, out, output_group, visibility):
    _filegroup = "{}-filegroup".format(name)

    native.filegroup(
        name = _filegroup,
        srcs = [src],
        output_group = output_group,
    )

    native.genrule(
        name = name,
        cmd = "cp $< $@",
        srcs = [_filegroup],
        outs = [out],
        visibility = visibility,
    )

copy_file = macro(
    implementation = _copy_file_impl,
    attrs = {
        "src": attr.label(
            mandatory = True,
            configurable = False,
        ),
        "out": attr.label(
            mandatory = True,
            configurable = False,
        ),
        "output_group": attr.string(
            mandatory = True,
            configurable = False,
        ),
    },
)