cue/load: support loading a CUE package whose directory ends with .cue

When we load a CUE instance, we need to traverse the chain of parent
directories up to the module root to potentially load parent packages
with the same package name, if there are any. As such, we need to
parse any CUE files in parent directories.

If we are using cue/load to load a CUE package in a directory whose name
looks like a CUE file, this would work with older CUE versions like
v0.7.1, but it fails on newer ones like v0.11.0. This is because the new
logic brought in with CUE_EXPERIMENT=modules read and parsed all files
from parent directories with a ".cue" extension, regardless of whether
they were a directory or not.

The old code before CUE_EXPERIMENT=modules used IsDir to filter out
directories, presumably because package directory names such as
taxes.cue from https://github.com/tmm1/taxes.cue do happen.
Do the same in the new code, which is easy enough.

The test is added in the very same commit, and not in a previous one,
because the mere addition of the testdir.cue testdata directory
causes all other tests, which load parent directories, to also fail.

Fixes #3592.

Signed-off-by: Daniel Martí <mvdan@mvdan.cc>
Change-Id: I10bc05c1f821444bd913e6fc694b3194efb6da8b
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1204614
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Reviewed-by: Roger Peppe <rogpeppe@gmail.com>

cue/load/loader_test.go

@@ -636,7 +636,21 @@ display:.
 files:
     $CWD/testdata/testmod/test.cue
 imports:
-    mod.test/test/sub: $CWD/testdata/testmod/sub/sub.cue`,
+    mod.test/test/sub: $CWD/testdata/testmod/sub/sub.cue`}, {
+		// This tests that we can load a CUE package by pointing Dir to it
+		// even when the package's directory name ends with ".cue".
+		name: "DirWithCUEFileExtension",
+		cfg: &Config{
+			Dir: filepath.Join(testdataDir, "testdir.cue"),
+		},
+		args: []string{"."},
+		want: `path:   mod.test/test/testdir.cue@v0:testdir
+module: mod.test/test@v0
+root:   $CWD/testdata/testmod
+dir:    $CWD/testdata/testmod/testdir.cue
+display:.
+files:
+    $CWD/testdata/testmod/testdir.cue/test.cue`,
 	}}
 	tdtest.Run(t, testCases, func(t *tdtest.T, tc *loadTest) {
 		pkgs := Instances(tc.args, tc.cfg)

cue/load/testdata/testmod/testdir.cue/test.cue

@@ -0,0 +1,3 @@
+package testdir
+
+"data from testdir"

internal/mod/modimports/modimports.go

@@ -106,6 +106,10 @@ func PackageFiles(fsys fs.FS, dir string, pkgQualifier string) func(func(ModuleF
 				if e.Name() == "cue.mod" {
 					inModRoot = true
 				}
+				if e.IsDir() {
+					// Directories are never package files, even when their filename ends with ".cue".
+					continue
+				}
 				pkgName, cont := yieldPackageFile(fsys, path.Join(dir, e.Name()), selectPackage, yield)
 				if !cont {
 					return