PkgCreator: ignore cwd for pkg_request relative dirs that might exist

- for 'resources', 'scripts', etc. that are using relative paths,
  modify find_path_for_relpath() so that it only searches the
  search order directories
- otherwise, os.path.exists(relpath) will return the relative path
  of something that may be found in the current working directory
  (for example, the 'Scripts' directory at the root of the autopkg
  Git repo, which is not related to the AutoPkg.pkg recipe)
PkgCreator: don't search RECIPE_CACHE_DIR for relative paths