internal/txtar: bring back to main repo, for tests in std
This used to be cmd/go/internal/txtar,
and then it was moved to golang.org/x/tools/txtar
and revendored from there into cmd/vendor/golang.org/x/tools/txtar.
We have a use for txtar in a new test in the standard library,
which cannot access cmd/vendor. But we also don't really want
to vendor it into the standard library as is, because that would
be the first vendoring of x/tools in std, and it would be better
to keep std separate from x/tools, even for testing.
Instead, since a little copying is better than a little dependency,
just make a copy in internal/txtar. The package does not change.
Having done that, replace the uses in cmd/go so that there's
only one copy in the main repo.