skargo: use per-unit state.db to avoid cache thrashing

[cristianoc]

Lib and test targets sharing the same build directory and relocation model would read/write the same state.db, causing cache thrashing. Include the package name and unit hash in the state.db filename to make it unique per compilation unit.

Fix #1119

[beauby]

My understanding is that the lib and test targets (for the same build profile) should be able to share the same cache, because the cache for the test target should be a superset of the cache for the lib target. Is building lib after test causing pessimistic evictions?

[cristianoc]

My understanding is that the lib and test targets (for the same build profile) should be able to share the same cache, because the cache for the test target should be a superset of the cache for the lib target. Is building lib after test causing pessimistic evictions?

The observation is that cargo check is significantly slower when a single cache file is used.
Not sure if it matters, but the test target has additional source files that are not present in lib.

[beauby]

@cristianoc Do you observe this behavior for all projects or for one in particular?

[beauby]

Might be related: #971.