Revert "Group Emacs packages by their source archives (ELPA, Marmalade, etc.)."

The grouping of packages by their respective repository causes too
much complexity when resolving duplicate packages by their version.

This patch reverts commit a5bfcb03592d56293eee261947b88e158ed66f88 and
introduces meta.repository which contains the name of the repository.

Signed-off-by: Moritz Ulrich <moritz@tarn-vedra.de>