for example, if building in src/lib/container,
objects go in obj/container/, so that 6g -Iobj
will find "container/vector".

install packages in hierarchy in $GOROOT.

this change only updates gobuild.
another change will have to update all
the sources to refer to "container/vector" etc
and regenerate all the Makefiles.

there are some pretty lame functions here
(e.g., Mkdir, Remove, the Getenv("PWD"))
but i will implement better ones in another CL.

R=r
DELTA=117  (99 added, 2 deleted, 16 changed)
OCL=27550
CL=27574