package: Get rid of separate mechanisms for opening packages.

package: Get rid of separate mechanisms for opening packages.

Remove u_pkg_new_by_name and u_pkg_new_by_file, replacing them with
one function, u_pkg_open, that takes just a package name.  Named files
can now be part of the package search just like any other package.  This
should allow cool things like listing dependency packages on the upkg
command line.