cmd/cue: improve error message for malformed module path

When a module file's module directive holds an invalid module
path, the error message did not include the file name where the
error was.

Example of previous error message:

   invalid module path "": path "" has no major version

New error message:

   cue.mod/module.cue: invalid module path: path "" has no major version

This fix applies to all the subcommands that use `readModuleFile`:
`cue mod edit`, `cue mod get`, and `cue mod resolve`.

Fixes #3259

Signed-off-by: Roger Peppe <[email protected]>
Change-Id: I175a234ddc26ce42a76602b23fbfabf59ec640ad
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1197923
TryBot-Result: CUEcueckoo <[email protected]>
Reviewed-by: Daniel Martí <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>

Author: rogpeppe

cmd/cue/cmd/testdata/script/modget_bad_modulepath.txtar

@@ -0,0 +1,11 @@
+# Check that the error message when there's a bad module
+# path includes the path to the offending file (issue 3259)
+
+! exec cue mod get example.com/foo
+cmp stderr want-stderr
+-- want-stderr --
+cue.mod/module.cue: invalid module path: path "" has no major version
+-- cue.mod/module.cue --
+module: ""
+language: version: "v0.9.2"
+

internal/mod/modload/tidy.go

@@ -141,7 +141,7 @@ func readModuleFile(fsys fs.FS, modRoot string) (module.Version, *modfile.File,
 	}
 	mainModuleVersion, err := module.NewVersion(mf.QualifiedModule(), "")
 	if err != nil {
-		return module.Version{}, nil, fmt.Errorf("invalid module path %q: %v", mf.QualifiedModule(), err)
+		return module.Version{}, nil, fmt.Errorf("%s: invalid module path: %v", modFilePath, err)
 	}
 	return mainModuleVersion, mf, nil
 }