Treat src=name filegroups as handcrafted targets

This is groundwork for allowlist v2, as such targets are still
effectively referencable via their file syntax; thus, rdep targets of
these filegroups should still be treated as convertible.

Fixes: 302025822
Test: Unit test update
Change-Id: I93445ff75e7c0570530725d086e47c1532833c06
diff --git a/bp2build/filegroup_conversion_test.go b/bp2build/filegroup_conversion_test.go
index cb2e207..9c49dac 100644
--- a/bp2build/filegroup_conversion_test.go
+++ b/bp2build/filegroup_conversion_test.go
@@ -40,7 +40,9 @@
     srcs: ["foo"],
 }
 `,
-		ExpectedBazelTargets: []string{}})
+		ExpectedBazelTargets:       []string{},
+		ExpectedHandcraftedModules: []string{"foo"}},
+	)
 }
 
 func TestFilegroupSameNameAsFile_MultipleFiles(t *testing.T) {