refactor(ebuild/portage_conf): whitespace consistency
refactor(ebuild/portage_conf): only loop once over repos
refactor(ebuild/profiles): simplify repo map setup for portage-2 profiles

This should allow profile tests to work as expected on non-Gentoo
systems. Previously, any test using the ProfileNode class would call the
'load_config' method which would fail due to the make.conf file being
missing on non-Gentoo systems. However, note that full portage-2 profile
tests still need to be written since we're not writing parent files
using repo aliases yet.

This also lets us remove the crufty caching workaround from
'load_config' since it shouldn't be called multiple times anymore.