guix package: Add 'install', 'remove', and 'upgrade' aliases.

guix package: Add 'install', 'remove', and 'upgrade' aliases.

* guix/scripts/install.scm, guix/scripts/remove.scm,
guix/scripts/upgrade.scm, tests/guix-package-aliases.sh: New files.
* Makefile.am (MODULES, SH_TESTS): Add them.
* po/guix/POTFILES.in: Add them.
* guix/scripts/package.scm (guix-package): Split with...
(guix-package*): ... this.  New procedure.
* doc/guix.texi (Invoking guix package): Document them.
(Binary Installation, Application Setup, Package Management)
(Packages with Multiple Outputs, Package Modules)
(X.509 Certificates, Installing Debugging Files): Use 'guix install' in
simple examples.
* etc/completion/bash/guix (_guix_complete): Handle "install", "remove",
and "upgrade".