Auto merge of #7137 - ehuss:dep-info-absolute-paths, r=alexcrichton

Fix some issues with absolute paths in dep-info files.

There were some issues with how #7030 was handling translating paths in dep-info files. The main consequence is that when coupled with https://github.com/rust-lang/rust/pull/61727 (which has not yet merged), the fingerprint would fail and be considered dirty when it should be fresh.

It was joining [`target_root`](https://github.com/rust-lang/cargo/blame/1140c527c4c3b3e2533b9771d67f88509ef7fc16/src/cargo/core/compiler/fingerprint.rs#L1352-L1360) which had 3 different values, but stripping [only one](https://github.com/rust-lang/cargo/blame/1140c527c4c3b3e2533b9771d67f88509ef7fc16/src/cargo/core/compiler/mod.rs#L323). This means for different scenarios (like using `--target`), it was creating incorrect paths in some cases. For example a target having a proc-macro dependency which would be in the host directory.

The solution here is to always use CARGO_TARGET_DIR as the base that all relative paths are checked against. This should handle all host/target differences.

The tests are marked with `#[ignore]` because 61727 has not yet merged.

This includes a second commit (which I can open as a separate PR if needed) which is an alternate solution to #7034. It adds dep-info tracking for registry dependencies. However, it tries to limit which kinds of paths it tracks. It will not track package-relative paths (like source files). It also adds an mtime cache to significantly reduce the number of stat calls (because every dependency was stating every file in sysroot).

Finally, I've run some tests using this PR with 61727 in rust-lang/rust. I can confirm that a second build is fresh (it doesn't erroneously rebuild). I can also confirm that the problem in https://github.com/rust-lang/rust/issues/59105 has *finally* been fixed!

My confidence in all this is pretty low, but I've been unable to think of any specific ways to make it fail. If anyone has ideas on how to better test this, let me know. Also, feel free to ask questions since I've been thinking about this a lot for the past few weeks, and there is quite a bit of subtle stuff going on.