internal/mod/modpkgload: symbolic links can be valid CUE files too

The test for whether there are any potential CUE files in a directory
was too naive when testing for file type: it should do what Go does [1]
and use the underlying information from symbolic links.

[1] https://github.com/golang/go/blob/fc51e5023ec99fee405ae61ee4ab2c8c9bc66f24/src/cmd/go/internal/imports/scan.go#L29

Fixes #3298

Signed-off-by: Roger Peppe <rogpeppe@gmail.com>
Change-Id: Id6cc3308e49c11c221e6af423c886050f6dbd5c6
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1198003
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>

cmd/cue/cmd/testdata/script/issue3298.txtar

@@ -0,0 +1,26 @@
+[!unix] skip
+exec cp x/x.cue_actual x/x.cue
+exec cue export
+cmp stdout want-stdout
+
+rm x/x.cue
+exec ln -s x.cue_actual ./x/x.cue
+exec cue export .
+
+-- want-stdout --
+{
+    "self": "x"
+}
+-- cue.mod/module.cue --
+module: "example.com"
+
+language: version: "v0.9.0"
+-- main.cue --
+package main
+
+import "example.com/x"
+x
+-- x/x.cue_actual --
+package x
+
+self: "x"

internal/mod/modpkgload/import.go

@@ -249,7 +249,20 @@ func isDirWithCUEFiles(loc module.SourceLoc) (bool, error) {
 		return false, err
 	}
 	for _, e := range entries {
-		if strings.HasSuffix(e.Name(), ".cue") && e.Type().IsRegular() {
+		if !strings.HasSuffix(e.Name(), ".cue") {
+			continue
+		}
+		ftype := e.Type()
+		// If the directory entry is a symlink, stat it to obtain the info for the
+		// link target instead of the link itself.
+		if ftype&fs.ModeSymlink != 0 {
+			info, err := fs.Stat(loc.FS, filepath.Join(loc.Dir, e.Name()))
+			if err != nil {
+				continue // Ignore broken symlinks.
+			}
+			ftype = info.Mode()
+		}
+		if ftype.IsRegular() {
 			return true, nil
 		}
 	}