Add `root_path` attribute for directory crate roots (#4142)

Allows specifying `root_path` when the crate root (`srcs` entrypoint) is
a directory artifact (`TreeArtifact`). This enables rules_rust targets
(`rust_library`, `rust_binary`, `rust_test`, etc.) to consume directory
artifacts produced by upstream rules and correctly construct the path to
the crate root (`lib.rs` / `main.rs`).

We use something like this internally to build standard libraries from
sources distributed as .zip source archives: a custom rule extracts the
contents into a generated directory artifact, which then the rust rules
can consume as sources.

This may be helpful to achieve the request from
https://github.com/bazelbuild/rules_rust/issues/3757, where the user is
trying to consume a directory of generated sources.

* Adds `root_path` attribute to `_COMMON_ATTRS` and `CrateInfo`
provider.
* Updates `rustc_compile_action` argument construction (`rustc.bzl`) to
join `crate_info.root.path` with `root_path` when
`crate_info.root.is_directory` is true.
* Adds a unit and build test case under `test/root_path/` using a custom
rule that packages its `srcs` as a directory artifact (`TreeArtifact`)
and compiles a `rust_library` and `rust_test` from it.

I have taken special care in the rustc argument construction to ensure
it is compatible with the new [bazel path remapping
guidelines](https://github.com/bazelbuild/bazel/discussions/22658).

While here, added a few checks in the rules for combinations of `srcs`,
`crate` and `crate_root` attributes that don't make sense and would
cause compilation to fail later anyways.