Add support for relative paths for exclusions

[dalemyers]

Includes already support relative paths, but excludes don't. This means if you did something like

targets:
  Foo:
    sources:
      path: ../Foo
      excludes:
        - ../Foo/Bar

then it wouldn't work. This fix corrects that issue.

[dalemyers]

Weird. The tests all passed locally. Maybe a weird caching thing. I'll take a look when I get a moment.