cmd/go/internal/modload: use a structured error for 'ambiguous import'
This consolidates the construction of 'ambiguous import' errors to a
single location, ensuring consistency, and lays the groundwork for
automatic resolution in the future.
While we're at it, change "found" to "found package" to try to make
the cause of the error clearer.
Updates #32128
Updates #27899
Change-Id: I14a93593320e5c60d20b0eb686d0d5355763c30c
Reviewed-on: https://go-review.googlesource.com/c/go/+/196298
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Jay Conrod <jayconrod@google.com>