cmd/api: omit outside dependencies when listing the packages in "std"

As of CL 251159, when 'go list -deps std' is run within GOROOT/src, it
treats the vendored external dependencies as real module dependencies,
not standard-library "vendor/" packages (which still exist in that
case, but are treated as distinct packages outside the "std" module).

Fixes #41358
Updates #30241

Change-Id: Ic23eae9829d90e74a340d49ca9052e9191597410
Reviewed-on: https://go-review.googlesource.com/c/go/+/254738


Run-TryBot: Bryan C. Mills <bcmills@google.com>
Trust: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>