Mix feature flags into fingerprint/metadata shorthash

Mix feature flags into fingerprint/metadata shorthash

Since building dependencies results in different libraries
depending on the feature flags, I added the feature flags
into the short_hash. This solves an issue when multiple crates
share a target directory or multiple targets share a common
library with divergent feature flag choice.

- Only link binaries, build scripts, and top level deps
- Handle dylibs differently (no metadata filename / linking)
- Fingerprint based on link_dst rather than file_stem.

This (sadly) limits the effects of dep caching to things
which don't produce a hard-link. Currently, this is only dependent
library crates. Stil a big win.