This change extends goinstall to support "magic" package names of the
form:
        <host>/<repo>.<vcs>/<path>

Where <host> is the hostname, <repo> the path to the repository, <vcs>
the type of vcs (git, hg, bzr or svn), and <path> is the path inside the
repository that contains the source code for the package.

For example: "example.com/pub/foo.hg/src" means download the Mercurial
repository at either pub/foo.hg or pub/foo from example.com and then
build and install the source files from src inside the repository
checkout.

Repositories on the built-in hostings sites (github, bitbucket,
launchpad and googlecode) must still use the old form (i.e.
github.com/xxx/yyy.git/src will be rejected).

R=adg, rsc
CC=golang-dev
https://golang.org/cl/4626064