x/vgo: implement modfetch/svnrepo

[rsc]

Added modfetch/gitrepo. Now we need modfetch/svnrepo too.
(See also #25074.)

[rsc]

Notes from a few weeks ago below, using svn.apache.org/repos/asf/subversion as an example.

func (r *repo) Tags(prefix string) ([]string, error) {
	svn list https://svn.apache.org/repos/asf/subversion/tags (tags instead of trunk)
}

func (r *repo) Stat(rev string) (*codehost.RevInfo, error) {
	svn log -l 1 --xml remote
}

func (r *repo) ReadFile(rev, file string, maxSize int64) ([]byte, error) {
	svn cat remote+file@rev
}

func (r *repo) ReadZip(rev, file string, maxSize int64) (zip io.ReadCloser, actualSubdir string, err error) {
	svn export -r rev remote+file tmpdir
	build zip file
}

func (r *repo) LatestAt(t time.Time, branch string) ([]byte, error) {
	svn log -l 1 -r {DATE} remote (sub trunk for branch?)
	svn log -l1 -r '{2018-01-01T00:00:00Z}' https://svn.apache.org/repos/asf/subversion/trunk
	why does this not work?
}

[rsc]

All that remains is #26092.