cmd/cue: fix a regression with cue get go dropping CUE files

In https://cuelang.org/cl/1196294 I refactored the code to swap
filepath.WalkDir for os.ReadDir, given that we didn't need to read
directory trees recursively. However, I failed to realise that
a `return nil` would be problematic when replacing a callback function
with a regular for loop, as it now stopped the entire parent func.

We didn't notice as the test used to happen to hide the bug;
the improved test now shows that the bug is fixed.

Fixes #3644.

Signed-off-by: Daniel Martí <mvdan@mvdan.cc>
Change-Id: Ia0ebafcf481402faf106af26fea507da98dae47c
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1207330
Reviewed-by: Roger Peppe <rogpeppe@gmail.com>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>

cmd/cue/cmd/get_go.go

@@ -595,7 +595,7 @@ func (e *extractor) importCUEFiles(p *packages.Package, dstDir, args string) err
 			path := filepath.Join(srcDir, name)
 			name, ok := strings.CutSuffix(name, ".cue")
 			if !ok {
-				return nil
+				continue
 			}
 			f, err := parser.ParseFile(path, nil, parser.PackageClauseOnly)
 			if err != nil {

cmd/cue/cmd/testdata/script/get_go_non_local.txtar

@@ -11,11 +11,8 @@ go mod tidy
 exec cue get go externalmod.test/mixfiles
 cmp cue.mod/gen/externalmod.test/mixfiles/code_one_go_gen.cue cue.mod/gen/externalmod.test/mixfiles/code_one_go_gen.cue.golden
 cmp cue.mod/gen/externalmod.test/mixfiles/code_two_go_gen.cue cue.mod/gen/externalmod.test/mixfiles/code_two_go_gen.cue.golden
-# TODO(mvdan): fix https://cuelang.org/issue/3644
-! exists cue.mod/gen/externalmod.test/mixfiles/config_one_gen.cue
-! exists cue.mod/gen/externalmod.test/mixfiles/config_two_gen.cue
-# cmp cue.mod/gen/externalmod.test/mixfiles/config_one_gen.cue cue.mod/gen/externalmod.test/mixfiles/config_one_gen.cue.golden
-# cmp cue.mod/gen/externalmod.test/mixfiles/config_two_gen.cue cue.mod/gen/externalmod.test/mixfiles/config_two_gen.cue.golden
+cmp cue.mod/gen/externalmod.test/mixfiles/config_one_gen.cue cue.mod/gen/externalmod.test/mixfiles/config_one_gen.cue.golden
+cmp cue.mod/gen/externalmod.test/mixfiles/config_two_gen.cue cue.mod/gen/externalmod.test/mixfiles/config_two_gen.cue.golden
 
 # Verify dependencies are as expected
 cmp go.mod go.mod.golden