lib: add pkg lookpath, and make gosh use it

Details:
- Moves lookpath functionality out of v.io/x/lib/envvar, to
a new package named v.io/x/lib/lookpath
- Updates gosh to use this package, using the dirs from the
Shell's PATH variable

MultiPart: 2/2

Change-Id: I1a357758183cccf3a950176a3bc5bdf48d7fb887
12 files changed