Fixed #26249 -- Fixed collectstatic crash for files in STATIC_ROOT referenced by absolute URL.

collectstatic crashed when:

* a hashing static file storage backend was used
* a static file referenced another static file located directly in
  STATIC_ROOT (not a subdirectory) with an absolute URL (which must
  start with STATIC_URL, which cannot be empty)

It seems to me that the current code reimplements relative path joining
and doesn't handle edge cases correctly. I suspect it assumes that
STATIC_URL is of the form r'/[^/]+/'.

Throwing out that code in favor of the posixpath module makes the logic
easier to follow. Handling absolute paths correctly also becomes easier.