txtar-addmod command

The txtar-addmod command adds a module as a txtar archive to the a testdata module directory as understood by the goproxytest package (see https://godoc.org/github.com/rogpeppe/go-internal/goproxytest).

Usage:

txtar-addmod dir path@version...

where dir is the directory to add the module to.

In general, it's intended to be used only for very small modules - we do not want to check very large files into testdata/mod.

It is acceptable to edit the archive afterward to remove or shorten files.