Rename libchamplainmm to champlainmm

To better match the libchamplain naming convention, libchamplainmm
becomes now champlainmm. The project and the library remain as
libchamplainmm, as in the case of libchamplian; but internal
structure moves to champlainmm prefixes, e.g. headers are now
installed in champlainmm/.