Add [TRACKED_NO_CRATE_HASH] and [SUBSTRUCT] directives

This is necessary for options that should invalidate the incremental
hash but *not* affect the crate hash (e.g. --remap-path-prefix).

This doesn't add `for_crate_hash` to the trait directly because it's not
relevant for *types*, only for *options*, which are fields on a larger
struct. Instead, it adds a new `SUBSTRUCT` directive for options, which
does take a `for_crate_hash` parameter.

- Use TRACKED_NO_CRATE_HASH for --remap-path-prefix
- Add test that `remap_path_prefix` is tracked
- Reduce duplication in the test suite to avoid future churn

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