cmd/doc: use go list to determine import path if it's missing

cmd/doc: use go list to determine import path if it's missing

cmd/doc uses go/build to get information about the packages it's
documenting. In some cases, go/build can return a build.Package that it
couldn't determine an import path for, in which case it sets the import
path to ".". This can happen for relative package paths in in a module:
for relative package paths we don't use the go command to get
information about the module and just open the source files directly
instead, and will be missing the import path. This is usually okay
because go doc doesn't need to print the import path of the package it's
documenting, but for go doc -http, we want to know the import path so we
can open the right page in the browser.

For #68106

Change-Id: Ifba92862ad01d8d63f531c2451f18db2b0d7a3e5
Reviewed-on: https://go-review.googlesource.com/c/go/+/674556
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
Reviewed-by: Michael Matloob <matloob@google.com>