Ignore path deps in metadata hash

[illicitonion]

They don't end up factoring into what we generate, so we pretend they don't exist. This avoids people who are using both Cargo and Bazel needing to spuriously repin just because they edited Cargo-only dependency information.

The first few commits are #2688

[UebelAndre]

Are path dependencies not desired?

• Allow path dependencies without having everything in one Cargo workspace #1773

If this support was implemented, would there be anything else being filtered out?

[illicitonion]

Are path dependencies not desired?

I'm actually not sure what this would reasonably look like. It feels like maybe they are via #2674 and maybe we can automate setting some overrides up for path dependencies, but it's not super clear to me what target we can practically implicitly add a dependency on from a generated file into the local workspace. If we were to implement that kind of support, we would want to factor the replacement target, rather than the path dep value, into the cache key.

If this support was implemented, would there be anything else being filtered out?

Yeah, we should probably be filtering out everything that doesn't factor into dep resolution. So we should be stripping out things like authors, description, documentation, readme, homepage, ... - all of these things will currently require a repin if they change, even though they're just metadata for Cargo which Bazel will completely ignore.