pmb.config.init: fix performance regression when selecting "none" UI
The ui-extras questions will attempt to find a postmarketos-ui-<ui>
package in pmaports. If the package does not exist as "root" APKBUILD
it currently attempts to parse all APKBUILDs in case it is somewhere
defined as a subpackage. This is really slow (up to 2-3 seconds),
which feels weird during "pmbootstrap init".
For the UI packages we specifically look for the root UI package, not the subpackage, so let's skip searching for subpackages in this case. This makes selecting the "none" UI nice and fast again.