dev/sg: clean up linter structure

Created by: bobheadxi

Gives linters a similar structure to checks, moving core types and utility funcs into a separate package and moving actual lint implementations to a separate file.

Also fixes a bug where linter flags weren't being propagated correctly

Test plan

sg update -local
sg lint

The help text has also improved by making sure ShortUsage is set and ShortHelp and LongHelp are both set to Help`