[GRASS5] gis.m: Loads as a package

Hi,

I made a package index for most of the files in gis.m. I swapped out most of
the "source"s in gm.tcl for lazy loading in the package. This makes the layer
types, for example, not be loaded until someone actually uses them. On my
reasonably slow and reasonably old computer gis.m now loads in just over 2
seconds if wish has been recently run (and thus its files are in memory
cache) or if it's currently running (no loading ".so"s).

--Cedric