install_package(pn, type, url, force)
Installs the package named pn. Type must be one of the symbols 'local,
'git or 'remote. If type is
'local, url must be the full path to the package directory or
the full path of a tar.gz, a tar.bz2 or a .zip file that contains the package. For type
'git, url must be the location of the git repository of the package.
If type is 'remote,
url must point to a tar.gz, a tar.bz2 or a .zip package hosted on an HTTP or
FTP server.
Return pn on success.