pmerge: check installed repos if input matches multiple pkgs

This makes pmerge work similar to portage where running `emerge pkgname`
will rebuild the installed pkg that matches the pkgname if multiple
matches exist and only one of them is installed.
add todo to support portage's keepdir naming scheme
remove unused module import
add support for COLLISION_IGNORE (see make.conf man page)

This fixes upgrading from old python eclass based pkgs to newer
python-r1 eclass pkgs. The newer eclasses include the compiled bytecode
and other generated files in installed files for pkgs while the older
eclasses didn't causing collision-protect failures when not respecting
the default COLLISION_IGNORE values from the base profile make.defaults.
add todo for UNINSTALL_IGNORE and protect-owned support