[FR]: mtree_spec should support $(locations) expansions

[thesayyn]

What is the current behavior?

Currently, there's no way to refer to generated files in tar(mtree=[]).

Describe the feature

Make this work;

tar(
    name = name, 
    srcs = [":content"],
    mtree = [
        "#mtree",
        "{} uid=0 gid=0 mode=0755 time=0 type=file content=$(location :content)".format(path.lstrip("/")),
    ],
    **common_kwargs
)

[gzm0]

Note that IIUC this is also necessary to be able to build tar from another repository:

This works when built in the main repository:

tar(
    name = "vite-config-tar",
    srcs = ["vite.config.js"], # just a source file
    mtree = [
        "app/vite.config.js uid=0 gid=0 mode=0755 type=file content=private/react_app/vite.config.js",
    ],
    visibility = ["//visibility:public"],
)

But fails when the target is built from another repository:

ERROR: /home/tos/.cache/bazel/_bazel_tos/9a3bb826a3e2f2e221c72da13796debf/external/dh_buildlib/private/react_app/BUILD.bazel:23:4: Tar external/dh_buildlib/private/react_app/vite-config-tar.tar failed: (Exit 1): bsdtar.sh failed: error executing command (from target @dh_buildlib//private/react_app:vite-config-tar) external/bsd_tar_linux_amd64/bsdtar.sh --create --file bazel-out/k8-fastbuild/bin/external/dh_buildlib/private/react_app/vite-config-tar.tar ... (remaining 1 argument skipped)

Use --sandbox_debug to see verbose messages from the sandbox and retain the sandbox build root for debugging
bsdtar: Error reading archive bazel-out/k8-fastbuild/bin/external/dh_buildlib/private/react_app/_vite-config-tar.mtree.txt: Can't open private/react_app/vite.config.js
bsdtar: Error exit delayed from previous errors.