cmd/go/internal/modfetch/codehost: add general vcs support

Add support for code fetch from bzr, fossil, hg, svn repositories.
Table-driven in a similar manner to old go get, but a different
operation set, so different code.

The git support remains a separate, special case, making use of
git-specific optimizations that wouldn't fit into the general
framework being introduced for the other version control systems.

Now vgo can handle launchpad.net/gocheck and bitbucket.org/ww/goautoneg.

Support for Subversion zip files remains a TODO (#26092).

Change-Id: I6f296a7f6588df08d48b39b98fe255d9b07c76d0
Reviewed-on: https://go-review.googlesource.com/120998
Reviewed-by: Bryan C. Mills <bcmills@google.com>