More bottling HOMEBREW_LIBRARY changes

- Refuse to create bottles which have non-relocatable references to
  `HOMEBREW_LIBRARY`. This allows us to make all bottles ignore where
  `HOMEBREW_REPOSITORY` is (even those that aren't `cellar :any`).
  I cannot see any circumstances in which any bottle should link to
  anything within `HOMEBREW_REPOSITORY`.
- Remove audit that becomes unnecessary given the above change.
- Relocate references to `@HOMEBREW_LIBRARY@` but don't actually write
  any references yet. This will allow us to move to using
  `@HOMEBREW_LIBRARY` and remove all relocation of `HOMEBREW_REPOSITORY`
  in a future release (2.7.1, most likely).