Only check `src` directory for changes

[nesl247]

It would be great if we could limit the scope in which it looks for files. So rather than looking at all files that end in .php for example, it would only look for src/**/*.php. This way configuration changes in /phinx.php, config/parameters.php, etc. type of files would not trigger the suggestion.

[maks-rafalko]

A simple solution would be to introduce sourceDir (similar to testDir from the config https://github.com/infection/tests-checker/blob/b7713f4a92258d123c9a5a7fc5e1f9e2af290033/src/index.ts#L9)

I don't think I'm able to do it in the near future, so PR is very welcome!