cue/load: add test coverage for loading a legacy cue.mod file

https://cuelang.org/cl/1188428 explicitly dropped support for legacy
cue.mod files in favor of cue.mod/module.cue files.
However, we did not add a test case for the legacy edge case,
which should give the new human-friendly error.

Signed-off-by: Daniel Martí <[email protected]>
Change-Id: I067556498086ceba16c9a94ede22b47abb5aa8f9
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1198064
Reviewed-by: Matthew Sackman <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>

Author: mvdan

cue/load/loader_test.go

@@ -539,7 +539,19 @@ root:   $CWD/testdata/testmod
 dir:    $CWD/testdata/testmod/tagswithimports
 display:.
 files:
-    $CWD/testdata/testmod/tagswithimports/nonprod.cue`}}
+    $CWD/testdata/testmod/tagswithimports/nonprod.cue`}, {
+		name: "ModuleFileNonDirectory",
+		cfg: &Config{
+			Dir: testdata("testmod_legacymodfile"),
+		},
+		args: []string{"."},
+		want: `err:    cue.mod files are no longer supported; use cue.mod/module.cue
+path:   ""
+module: ""
+root:   ""
+dir:    ""
+display:""`},
+	}
 	tdtest.Run(t, testCases, func(t *tdtest.T, tc *loadTest) {
 		pkgs := Instances(tc.args, tc.cfg)
 

cue/load/testdata/testmod_legacymodfile/cue.mod

@@ -0,0 +1 @@
+module: "test.example/foo"

cue/load/testdata/testmod_legacymodfile/foo.cue

@@ -0,0 +1,2 @@
+package foo
+