Allow managing pkgset under local folder.

While replacing pakcageset name with "--local", the packageset
information will be managed under some/local/repo/.gvm_local folder, and
add the root path of repository as the first path in GOPATH and PATH.
This makes the repository more portable since we don't store all the
go-installed package files under $HOME/.gvm .

The repository root path for gvm is definded as $PWD while
"pkgset create --local" . When "pkgset list" under some configured local
repository, it will list the root path as the first packageset, and
marked with "L".