Add --sync-submodule <glsa|news|profiles> (work in progress)

This adds support for a new --sync-submodule option to both emerge and
emaint. When this option is used with the sync action, only the selected
submodules are synced. Each submodule is referenced using an abstract
identifier, which serves to hide the implementation details involving
the locations of specific submodules within each repository.

X-Gentoo-Bug: 89641
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=89641