tools/veyron: Set VDLPATH envvar, currently the same as GOPATH.
tools/veyron: Set VDLPATH envvar, currently the same as GOPATH.

I have a subsequent change which will cause vdl to start using
VDLPATH rather than GOPATH.  These steps are prerequisites for
our eventual goal of moving .vdl files out of the go repos.

Also a minor refactoring of VeyronEnvironment to keep it a
reasonable size.

Change-Id: Ib04938ac56a56678aca8976109757c81eacdf95e

vanadium/go.devtools,vanadium/go.devtools,vanadium/go.devtools