cue/load: fix @if attributes in command line-specified files

We were treating files directly specified on the command
line as outside of the module when actually they should not be.

Fixed #3250

Signed-off-by: Roger Peppe <[email protected]>
Change-Id: I6b6649aa0e53ed0184a2b3d0648f86bbd98561ed
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1202738
Reviewed-by: Daniel Martí <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1202873

Author: rogpeppe

Diff for: cmd/cue/cmd/testdata/script/issue3250.txtar

@@ -10,9 +10,9 @@ exec cue mod init
 # passes - with module
 exec cue export -t foo .:foo
 
-# fails - with module
-# TODO fix this case
-! exec cue export -t foo foo.cue
+# passes - with module
+# (failed previously due to #3250).
+exec cue export -t foo foo.cue
 
 -- foo.cue --
 @if(foo)

Diff for: cue/load/loader_common.go

@@ -270,9 +270,11 @@ func (fp *fileProcessor) add(root string, file *build.File, mode importMode) {
 
 	if !fp.c.AllCUEFiles {
 		tagIsSet := fp.tagger.tagIsSet
-		if p.Module != fp.c.Module {
+		if p.Module != "" && p.Module != fp.c.Module {
 			// The file is outside the main module so treat all build tag keys as unset.
-			// TODO also consider packages explicitly specified on the command line.
+			// Note that if there's no module, we don't consider it to be outside
+			// the main module, because otherwise @if tags in non-package files
+			// explicitly specified on the command line will not work.
 			tagIsSet = func(string) bool {
 				return false
 			}