union: Don't traverse sub-directories only found in one element of the union.

This significantly reduces I/O when building profiles, especially with
lots of package-specific sub-directories (such as 'share/emacs/24.3',
'texmf', etc.)

* guix/build/union.scm (union-build)[file-tree](others-have-it?): New
  procedure.  Use it in the 'enter?' parameter of 'file-system-fold';
  change 'skip' parameter accordingly.
* tests/union.scm ("union-build"): Ensure that 'include' is a symlink
  and 'bin' is a directory.

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