cmd/go/internal/modload: improve error message for failing to read module listed in go.work

Run "go build ./x" in this workspace:

  -- go.work --
  use ./y
  -- x/go.mod --
  module x

  go 1.19
  -- x/m.go --
  package m

It fails with: "go: open /tmp/foo/y/go.mod: no such file or directory".
It's unclear where the name "y" comes from.
This change will emit error like: "go: cannot load module listed in
go.work file: open /tmp/foo/y/go.mod: no such file or directory"

Fixes #55952.

Change-Id: Ia45dd915e3fbd6e33340f352b3d6235c6c31190b
GitHub-Last-Rev: 410de1b
GitHub-Pull-Request: #56050
Reviewed-on: https://go-review.googlesource.com/c/go/+/438147
Run-TryBot: hopehook <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Auto-Submit: Bryan Mills <[email protected]>
Reviewed-by: Bryan Mills <[email protected]>
Run-TryBot: Bryan Mills <[email protected]>
Reviewed-by: Alan Donovan <[email protected]>

Author: ZekeLu

src/cmd/go/internal/modload/init.go

@@ -718,7 +718,11 @@ func LoadModFile(ctx context.Context) *Requirements {
 		var fixed bool
 		data, f, err := ReadModFile(gomod, fixVersion(ctx, &fixed))
 		if err != nil {
-			base.Fatalf("go: %v", err)
+			if inWorkspaceMode() {
+				base.Fatalf("go: cannot load module listed in go.work file: %v", err)
+			} else {
+				base.Fatalf("go: %v", err)
+			}
 		}
 
 		modFiles = append(modFiles, f)

src/cmd/go/testdata/script/work_use_issue55952.txt

@@ -0,0 +1,11 @@
+! go list .
+stderr '^go: cannot load module listed in go\.work file: open .+go\.mod:'
+
+-- go.work --
+use ./y
+-- x/go.mod --
+module x
+
+go 1.19
+-- x/m.go --
+package m