internal/mod/modimports: exclude hidden CUE files

The `modimports` logic should exclude "hidden" CUE files
just as it excludes directories.

For #3795

Signed-off-by: Roger Peppe <[e-mail]>
Change-Id: I9219cd58cf7a02adaed5774af71669596849832e
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1211242
Reviewed-by: Daniel Martí <[e-mail]>
TryBot-Result: CUEcueckoo <[e-mail]>
Unity-Result: CUE porcuepine <[e-mail]>

cmd/cue/cmd/testdata/script/eval_hidden.txtar

···
 cmp stdout expect-stdout-underscore
 
 # Ensure package eval excludes both for completeness
-# TODO fix this
-! exec cue eval .
-cmp stderr expect-stderr-pkg
-# cmp stdout expect-stdout-pkg
+exec cue eval .
+cmp stdout expect-stdout-pkg
 
 -- .foo.cue --
 package foo
···
 b: 42
 -- expect-stdout-pkg --
 c: 42
--- expect-stderr-pkg --
-found packages "bar" (.bar.cue) and "foo" (.foo.cue) in "."

internal/mod/modimports/modimports.go

···
 					// Directories are never package files, even when their filename ends with ".cue".
 					continue
 				}
+				if isHidden(e.Name()) {
+					continue
+				}
 				pkgName, cont := yieldPackageFile(fsys, path.Join(dir, e.Name()), selectPackage, yield)
 				if !cont {
 					return
···
 	// in subdirectories.
 	for _, entry := range entries {
 		if entry.IsDir() {
+			continue
+		}
+		if isHidden(entry.Name()) {
 			continue
 		}
 		fpath := path.Join(fpath, entry.Name())
···
 		if !entry.IsDir() {
 			continue
 		}
-		if name == "cue.mod" || strings.HasPrefix(name, ".") || strings.HasPrefix(name, "_") {
+		if name == "cue.mod" || isHidden(name) {
 			continue
 		}
 		fpath := path.Join(fpath, name)
···
 	pf.Syntax = syntax
 	return syntax.PackageName(), yield(pf, nil)
 }
+
+func isHidden(name string) bool {
+	return name == "" || name[0] == '.' || name[0] == '_'
+}

internal/mod/modimports/testdata/simple.txtar

···
-TODO fix code so it does not include "hidden" CUE files
 -- want --
 x.cue "dep1" "dep2:a" "dep2:b" "dep3"
 z.cue
-y/.omitted.cue
-y/_omitted.cue
 y/y1.cue "dep3" "dep4"
 y/y2.cue
 y/z1.cue