portageq: remove unused imports
portageq: emulate subset of Pkgcore's pquery

Pkgcore pquery compatible options:

Repository matching options:
  --repo=REPO           repo to use (default is PORTDIR if omitted)
  --all-repos           search all repos

Package matching options:
  --herd=HERD           exact match on a herd
  --maintainer-email=MAINTAINER_EMAIL
                        comma-separated list of maintainer email regexes to
                        search for

Output formatting:
  -n, --no-version      collapse multiple matching versions together
portageq: support atoms for pquery mode
portageq: visiblity filter for pquery mode
portageq: optimize cat/name expansion for pquery
insert_category_into_atom: handle wildcard
portageq: add pquery to global command list
portageq: fix visiblity filter with no atoms
portageq: optimize pquery --no-version
resolver/output: unicode(), not str() for python2
resolver/output: rename Display._blockers() param
Display._blockers(): distinguish hard blocking
display_preserved_libs: distinguish prsvd cnsmrs

This will fix bug #461908.