Implement a dedicated `swift test codecov` command

[grynspan]

Tracks work to implement a dedicated code coverage subcommand of swift test that can:

1. Show the path to code coverage files (existing --show-codecov-path option)
2. Collate .profraw files on demand
3. Present insights from code coverage (e.g. percentages of modules/files covered) in a human-readable form

[grynspan]

@Sajjon has done some work in #7497 in this direction.