build packages in obj/ subdirectory that mimics $GOROOT/pkg.
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.