cmd/go: eliminate empty '()' when passing -mod=readonly explicitly to 'go list'

Discovered while investigating #37197.

Updates #33326
Updates #34822

Change-Id: I38b136a4ee762a580a554125066b9778491295f2
Reviewed-on: https://go-review.googlesource.com/c/go/+/219237
Run-TryBot: Bryan C. Mills <[email protected]>
Reviewed-by: Michael Matloob <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>

Author: Bryan C. Mills

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

@@ -184,8 +184,9 @@ func Import(path string) (m module.Version, dir string, err error) {
 		if !pathIsStd {
 			if cfg.BuildModReason == "" {
 				queryErr = fmt.Errorf("import lookup disabled by -mod=%s", cfg.BuildMod)
+			} else {
+				queryErr = fmt.Errorf("import lookup disabled by -mod=%s\n\t(%s)", cfg.BuildMod, cfg.BuildModReason)
 			}
-			queryErr = fmt.Errorf("import lookup disabled by -mod=%s\n\t(%s)", cfg.BuildMod, cfg.BuildModReason)
 		}
 		return module.Version{}, "", &ImportMissingError{Path: path, QueryErr: queryErr}
 	}

src/cmd/go/testdata/script/mod_readonly.txt

@@ -7,6 +7,7 @@ go mod edit -fmt
 cp go.mod go.mod.empty
 ! go list all
 stderr '^can''t load package: x.go:2:8: cannot find module providing package rsc\.io/quote: import lookup disabled by -mod=readonly'
+! stderr '\(\)' # If we don't have a reason for -mod=readonly, don't log an empty one.
 cmp go.mod go.mod.empty
 
 # -mod=readonly should be set implicitly if the go.mod file is read-only