minor comment spelling fixes
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.
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 various todos