fix: persist original file path when using bases (#672)

* fix: persist original file path when using bases

Prior to this change, the resulting lock file was called `<bases[0]>.lock`,
instead of `<filename>.lock`.

This change ensures the final, merged state has the correct `.FilePath`.

* test: Assert proper FilePath in layered HelmState

pkg/app/app_test.go

@@ -1127,6 +1127,10 @@ helmDefaults:
 	if st.Releases[1].Values[0] != "{{`{{.Release.Name}}`}}/values.yaml" {
 		t.Errorf("unexpected releases[0].missingFileHandler: expected={{`{{.Release.Name}}`}}/values.yaml, got=%s", st.Releases[1].Values[0])
 	}
+
+	if st.FilePath != yamlFile {
+		t.Errorf("unexpected filePath: expected=%s, got=%s", yamlFile, st.FilePath)
+	}
 }
 
 func TestLoadDesiredStateFromYaml_MultiPartTemplate(t *testing.T) {