Make normalized_path use prefixed_path

Currently collectfast ignores the prefix on source storages when looking up etags.
So if you have in settings something like:
STATICFILES_DIRS = [
    os.path.join(PACKAGE_ROOT, "static"),
    ('ng', os.path.join(PACKAGE_ROOT, "angular/app")),
]
etag lookups on all static files within angular/app fail and the files are re-copied to STATIC_ROOT/ng/ every time.

In this case when copying PACKAGE_ROOT/angular/app/index.html, path='index.html' and prefixed_path='ng/index.html'
(source_storage is a FileSystemStorage object with prefix='ng')