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: 410de1b4a7
GitHub-Pull-Request: golang/go#56050
Reviewed-on: https://go-review.googlesource.com/c/go/+/438147
Run-TryBot: hopehook <hopehook@golangcn.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>

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

@@ -718,8 +718,12 @@ func LoadModFile(ctx context.Context) *Requirements {
 		var fixed bool
 		data, f, err := ReadModFile(gomod, fixVersion(ctx, &fixed))
 		if err != nil {
+			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)
 		mainModule := f.Module.Mod

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