Previously, each package in the particle was analyzed on its own and
spawning portage against multiple packages was not possible.
This commit introduces the ability to aggregate packages in particles
in "groups", so that they can be considered together in the same
portage merge transaction.
The previously exported environment variable MATTER_PACKAGE_NAME has
been renamed into MATTER_PACKAGE_NAMES and contains a space separated
list of package dependencies that are in the merge transaction.
If SPM repository changes are not allowed when a package needs to be bumped,
the package won't be bumped/recompiled.
However, there is a case in where the SPM repo change constraint loses its
importance, and it's when the current SPM repository no longer contains the
target package (thus, changes have been upstreamed or they're just not needed
anymore).
Using "spm-repository-change-if-upstreamed: yes" makes possible to ignore
SPM repo changes in this case.
The default behaviour does not change, spm-repository-change-if-upstreamed is
disabled by default (== no as default config value).
