buck2 ocaml ide output: enable --show-output and be more consistent

Summary:
## What

Use `DefaultInfo` instead of a custom provider type for the `.cmt` and `.cmti` files we produce for IDE support.

## Why

D52840634 introduced the ability to build IDE outputs without linking, but had the following downsides:
- `--show-output` and `--show-full-output` didn't show anything for IDE targets, which impaired debuggability
- (warning: long explanation, skippable:) I didn't make the change to use `OCamlIdeInfo` all OCaml rules, which was inconsistent and probably didn't avoid all linking. When I did try to make the change for all OCaml IDE rules, the expected .cmt files were not generated. I suspect a target needs to have a default_output in order for this line of our BXL for generating build artifacts to do anything:

https://www.internalfb.com/code/fbsource/[3905ae9c88fb4245b97cbac20c5a387c1f816655]/fbcode/common/ocaml/gen_merlin.bxl?lines=18-21

Reviewed By: ndmitchell

Differential Revision: D52952776

fbshipit-source-id: 9409ef710d6f4def1a7a4ed5ebf280850fa4d169

prelude/ocaml/ocaml.bzl

@@ -96,7 +96,7 @@ load("@prelude//utils:graph_utils.bzl", "breadth_first_traversal", "post_order_t
 load("@prelude//utils:platform_flavors_util.bzl", "by_platform")
 load("@prelude//utils:utils.bzl", "filter_and_map_idx", "flatten")
 load(":makefile.bzl", "parse_makefile")
-load(":ocaml_toolchain_types.bzl", "OCamlLibraryInfo", "OCamlLinkInfo", "OCamlToolchainInfo", "OcamlIdeInfo", "OtherOutputsInfo", "merge_ocaml_link_infos", "merge_other_outputs_info")
+load(":ocaml_toolchain_types.bzl", "OCamlLibraryInfo", "OCamlLinkInfo", "OCamlToolchainInfo", "OtherOutputsInfo", "merge_ocaml_link_infos", "merge_other_outputs_info")
 
 BuildMode = enum("native", "bytecode", "expand")
 
@@ -688,8 +688,9 @@ def ocaml_library_impl(ctx: AnalysisContext) -> list[Provider]:
     other_outputs_info = merge_other_outputs_info(ctx, other_outputs, _attr_deps_other_outputs_infos(ctx))
 
     info_ide = [
-        OcamlIdeInfo(
-            outputs = [cmd_args(other_outputs_info.info.project_as_args("ide"))],
+        DefaultInfo(
+            default_output = cmts_nat[0] if cmts_nat else None,
+            other_outputs = [cmd_args(other_outputs_info.info.project_as_args("ide"))],
         ),
     ]
     info_byt = [
@@ -783,6 +784,7 @@ def ocaml_binary_impl(ctx: AnalysisContext) -> list[Provider]:
 
     info_ide = [
         DefaultInfo(
+            default_output = cmts_nat[0] if cmts_nat else None,
             other_outputs = [cmd_args(other_outputs_info.info.project_as_args("ide"))],
         ),
     ]
@@ -874,7 +876,7 @@ def ocaml_object_impl(ctx: AnalysisContext) -> list[Provider]:
 
     info_ide = [
         DefaultInfo(
-            default_output = obj,
+            default_output = cmts[0] if cmts else None,
             other_outputs = [cmd_args(other_outputs_info.info.project_as_args("ide"))],
         ),
     ]
@@ -956,7 +958,7 @@ def ocaml_shared_impl(ctx: AnalysisContext) -> list[Provider]:
 
     info_ide = [
         DefaultInfo(
-            default_output = binary_nat,
+            default_output = cmts_nat[0] if cmts_nat else None,
             other_outputs = [cmd_args(other_outputs_info.info.project_as_args("ide"))],
         ),
     ]

prelude/ocaml/ocaml_toolchain_types.bzl

@@ -58,12 +58,6 @@ OCamlLinkInfo = provider(
     fields = {"info": provider_field(typing.Any, default = None)},
 )
 
-OcamlIdeInfo = provider(
-    fields = {
-        "outputs": provider_field(typing.Any, default = []),
-    },
-)
-
 # A record of an OCaml library.
 OCamlLibraryInfo = record(
     # The library target name: e.g. "`foo`"