Add unused-imports tool

[keith]

This tool uses the index data to suggest imports that can be removed
from your files. Currently compared to SwiftLint's implementation it's
much faster, but doesn't yet add missing imports, or remove unused
system imports.

It's currently lightly affected by upstream issues listed here
#5

Todos:

• Need to add some way to ignore files and maybe modules, we do this for generated files we don't want the tool to analyze, and third parties that we pull in
• Potentially we want to add missing imports as well as removing invalid ones. this might be required because if you remove an invalid import that was polluting one you actually need, if you don't add the needed one here it will require manual work to add instead, also swiftlint does this so otherwise it's a bit of a regression
• handle system imports, right now the indexes ive been testing with don't even have info for system modules, so maybe it would just work if i included those as well, but i believe swiftlint handles this too, so otherwise it would be a regression, although the value of removing system imports is much lower than removing your own imports
• Test against swiftlint's removals, I haven't tested to see if this has any false-negatives that swiftlint doesnt
• FIXMEs in code
• Parallelize (probably v2)

[keith]

cc @jpsim @codeman9 @marcosgriselli

[keith]

for comparison we run swiftlint's logic sharded and it takes ~15 hours of machine time. The current implementation here takes ~20 seconds

[keith]

Ok i think this is good enough to merge, the checks that aren't done can be done as follow ups and will be more understandable at this point. if you try this and find any issues let me know!