cmd/go/internal/modload: add an Unwrap method on ImportMissingError

Jay suggested this in CL 189780, and it seems semantically correct.

As far as I can tell this has no impact one way or the other right
now, but might prevent confusion (or at least give us more experience
with error handling!) in future changes.

Updates #30748
Updates #28459
Updates #30322

Change-Id: I5d7e9a08ea141628ed6a8fd03c62d0d3c2edf2bb
Reviewed-on: https://go-review.googlesource.com/c/go/+/194817
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

src/cmd/go/internal/modload/import.go

@@ -48,6 +48,10 @@ func (e *ImportMissingError) Error() string {
 	return fmt.Sprintf("missing module for import: %s@%s provides %s", e.Module.Path, e.Module.Version, e.ImportPath)
 }
 
+func (e *ImportMissingError) Unwrap() error {
+	return e.QueryErr
+}
+
 // Import finds the module and directory in the build list
 // containing the package with the given import path.
 // The answer must be unique: Import returns an error