Allow packages to register themselves via entry_points.

Objective: Allow packages to be registered even if installed via a non-executable format (such as a Wheel).

Initially I had planned for each package to install a special file which could be modified by both the package itself and cmmnbuild-dep-manager, but it turned out that this was made more complex by the fact that files installed into $PREFIX/share are not installed with a development installation, making the whole thing much harder to support package development. Hence entrypoints.

CC: @rdemaria