cmd/doc: understand vendor directories in module mode
This change employs the same strategy as in CL 203017
to detect when vendoring is in use, and if so treats
the vendor directory as a (non-module, prefixless) root.
The integration test also verifies that the 'std' and 'cmd'
modules are included and their vendored dependencies are
visible (as they are with 'go list') even when outside of
those modules.
Fixes #35224
Change-Id: I18cd01218e9eb97c1fc6e2401c1907536b0b95f7
Reviewed-on: https://go-review.googlesource.com/c/go/+/205577
Run-TryBot: Bryan C. Mills <bcmills@google.com> Reviewed-by: Jay Conrod <jayconrod@google.com>