Don't perform partial filename matching.

When scanning for external files the FileScanner class used to perform a
fuzzy/partial filename match. This was not intended, instead it was supposed to
match entire filename segments.

For example, a value of "foo.rb" would match a path "/baz/bar_foo.rb" whereas
it should've only matched "/baz/foo.rb".

In a way this solves the problem described in #105 (at least that specific
case), I still have to figure out exactly what actually broke things.