conch_parser: restore building the Python extension

Summary:
On macOS the exported symbol has a leading underscore. Change buck config to
fix the build.

See the Python stdlib `_sqlite3` for example:

  $ nm -gU /opt/homebrew/Cellar/python38/3.8.3_1/Frameworks/Python.framework/Versions/3.8/lib/python3.8/lib-dynload/_sqlite3.cpython-38-darwin.so
  0000000000005132 T _PyInit__sqlite3
  ...

Reviewed By: DurhamG

Differential Revision: D35829957

fbshipit-source-id: 23259b982313baae01db0dc6ad4cbe79747e9766

eden/scm/setup.py

@@ -1786,6 +1786,11 @@ hgmainfeatures = (
     or None
 )
 rustextmodules = [
+    RustExtension(
+        "conch_parser",
+        package="",
+        manifest="edenscmnative/conch_parser/Cargo.toml",
+    )
 ]
 rustextbinaries = [
     RustBinary(