guix-package: Gracefully handle multiple installs of the same path.

* guix-package.in (guix-package)[process-actions]: Compute PACKAGES such
  that packages listed in INSTALL* are first removed from the remainder
  of the list.  When PROF is equal to the previous profile's store path,
  do nothing.  Reported by Andreas Enge <andreas@enge.fr>.
* tests/guix-package.sh: Test the behavior of installing the same store
  path twice.  When removing a package, omit its version number.