Sort CustomScalarImports

Summary:
* `meerkat` was emitting a custom scalar import out of order. This will hopefully fix it!

* We've seen instances of `CustomScalarImport`'s being emitted in different orders, though using the same source file. Sorting the `CustomScalarImport` before printing it might remove this inconsistent behavior.
* In particular, there is an instance of a custom scalar that is currently in reverse-alphabetical order in the checked-in generated artifact; this artifact is the one gets generated inconsistently. This diff changes that generated artifact, increasing my confidence that this will help.

Reviewed By: kassens

Differential Revision: D43368527

fbshipit-source-id: 500715406a141363f1c9a7b4243605fbe239ebb2

compiler/crates/relay-typegen/src/write.rs

@@ -962,7 +962,7 @@ fn write_custom_scalar_imports(
     custom_scalars: CustomScalarsImports,
     writer: &mut Box<dyn Writer>,
 ) -> FmtResult {
-    for (name, path) in custom_scalars.iter() {
+    for (name, path) in custom_scalars.iter().sorted_by_key(|(key, _)| *key) {
         writer.write_import_type(&[name.lookup()], path.to_str().unwrap())?
     }