guix package: Add '--allow-collisions'.

Fixes <https://bugs.gnu.org/30830>.
Suggested by Ricardo Wurmus <rekado@elephly.net>.

* guix/scripts/package.scm (build-and-use-profile): Add
 #:allow-collisions? and pass it to 'profile-derivation'.
(show-help, %options): Add '--allow-collisions'.
(manifest-action, process-actions): Pass #:allow-collisions? to
'build-and-use-profile'.
* tests/guix-package.sh: Add collision test.
* doc/guix.texi (Invoking guix package): Document '--allow-collisions'.