Fix scanning for dashed directories.

The FileScanner class wasn't properly scanning for files located in a directory
that used dashes instead of underscores. This in turn would prevent ruby-lint
from analysing itself due to the usage of lib/ruby-lint (instead of
lib/ruby_lint).

With this commit I've also taken the liberty of cleaning up the tests for
FileScanner. These tests now test more specific parts instead of only testing
the scan method.