encode as strings instead of SimpleElement

Summary:
the pyre client is erroring when json encoding this new argument. see test plan for details. This diff changes the argument from a SimpleElement (can't be encoded) to a string (can be encoded).

to do this we use the [command_line_argument](https://fburl.com/code/s82mhdzk) encoding function like in other places in the file.

Reviewed By: connernilsen

Differential Revision: D50748733

fbshipit-source-id: b9955d27e5ebc22dce842301ecf7b0698626898d

client/backend_arguments.py

@@ -127,7 +127,12 @@ def serialize(self) -> Dict[str, object]:
             **(
                 {}
                 if targets_fallback_sources is None
-                else {"targets_fallback_sources": targets_fallback_sources}
+                else {
+                    "targets_fallback_sources": [
+                        element.command_line_argument()
+                        for element in targets_fallback_sources
+                    ],
+                }
             ),
             **({} if mode is None else {"mode": mode}),
             **(

client/tests/backend_arguments_test.py

@@ -120,7 +120,7 @@ def test_serialize_source_paths(self) -> None:
                 "source_root": "/source",
                 "artifact_root": "/artifact",
                 "targets": ["//foo:bar"],
-                "targets_fallback_sources": [search_path.SimpleElement("/source")],
+                "targets_fallback_sources": ["/source"],
                 "mode": "opt",
                 "isolation_prefix": ".lsp",
                 "bxl_builder": "//foo.bxl:build",