LocationsManager: enable user package.bashrc

In /etc/portage/profile, we already have support for using directories
in the place of regular config files (portage1_directories = True). So,
go ahead and enable package.bashrc there too.

We could also support /etc/portage/package.bashrc, but that would be
more complicated since /etc/portage/bashrc is the long-standing location
of the user's bashrc which applies to all packages, and package.bashrc
would instead expect /etc/portage/bashrc to be a directory containing
per-package bashrc files. So for now, we only support package.bashrc
inside /etc/portage/profile.

Note that since /etc/portage/profile is not part of a repository,
there's no associated layout.conf. So, there's no way to enable the
profile-env profile-formats setting in /etc/portage/profile, except by
hardcoding.

Signed-off-by: Zac Medico <zmedico@gentoo.org>
Acked-by: Brian Dolbec <dolsen@gentoo.org>