This allows to tweak via extractor options some aspects of the cargo
configuration:
* the target architecture
* features (including `*` for all, which we must understand whether to
set by default)
* cfg overrides
Integration tests will be added in a follow-up commit.
This makes the clap flags overlay over `Config` entirely derived via an
attribute macro. Also, the `--intputs-file` option is replaced by a more
standard and versatile `@` parameter file mechanism.
The `.cargo/config.toml` override based workaround wasn't really
working, as while `cargo build|check` was reading that, `cargo metadata`
wasn't, ending up in a completely broken IDE experience.
For the moment, we just use a unified workspace `Cargo.toml` for all
extractors using the shared tree-sitter code, which has the downside of
making bazel pull in dependencies for all of them, and not being able to
do sparse checkouts for them. We should investigate and rivist this in
the future.
