perf: (issue #377) add caching and use re2 for alias scanning in files

[khoaHyh]

Attempting to continue the work done in #408. Closes #377.

Changes

• use caching to optimize file globbing and regex compilation
• swap stdlib regexp out for go-re2 for improved regex compilation and pattern matching
• added a test to assert alias-finding logic works properly when given a set of files with flag references in them

Benchmarks

I compiled and executed locally using local-find-code-refs --debug --dryRun -t=$SOME_TOKEN -d=. -p=default -r=some-repo --ignoreServiceErrors

Ran this on a repo where the command execution never completed (waited >30 minutes until I cancelled) and when using this branch's version, the command was able to complete within 8m14s.