portageq repositories_configuration: Require EROOT argument.
config: add PORTDIR to expand_map

Since commit a5dd8b59fbf253a435e5ca32c5fd0d128d1d8cfb, there was no
longer a default PORTDIR setting when loading make.conf. Now, the
default main-repo location will serve as the default PORTDIR setting,
for backward compatibility.
portageq: Use uses_eroot decorator.
portageq: Delete incorrect usage of uses_eroot decorator for pquery.