bazeloutputgroups.txt

I do have one question that I haven't answered yet which is: are OutputGroups applied to dependencies or do they only apply the target being run and its dependencies aren't changed? I think it's the latter which is going to make this a lil annoying because of all of the `transitive_$WHATEVER` variables kicking around.

Anyway, here's as far as a I got:

I thought I could use OutputGroups and DefaultInfo at the very top of the stack, such that _go_binary_impl, etc. would look something like:

```
def _go_binary_impl(ctx):
  """go_binary_impl emits actions for compiling and linking a go executable."""
  default = _go_binary_impl_more(ctx)
  race =_go_binary_impl_more(ctx, "_race", addl_gc_goopts=["-race"], addl_gc_linkopt=["-race"])
  return [DefaultInfo(files = depset([default])), OutputGroupInfo(race = depset([race]))]
```

That `_go_binary_impl_more` function (and the others like it) makes some changes to the labels and output file paths. It returns structs with the same fields that the original version of those `_impl` functions did.

However, DefaultInfo and OutputGroupInfo don't work in those cases because `depset` requires immutable objects and those structs returned by the `_more` functions are not immutable.

Further, from the limited documentation and my understanding of skylark, it seems you can only set the 4 fields `files`, `runfiles`, `data_runfiles`, and `default_runfiles`. Which means we couldn't return DefaultInfo and OutputGroupInfo from the `_impl` functions anyway, because we need additional providers like `cgo_object` and many more.

It's not yet clear to me if you can do things like this (supposing we're returning this from   `_go_binary_impl`):

```
  return struct(
    files = depset([DefaultInfo(files=depset[output_executable]), OutputGroup(files=depset([output_executable_race]))])
    runfiles = depset([DefaultInfo(files=depset[lib_result.runfiles]), OutputGroup(files=depset([lib_result_race.runfiles]))])
    # cgo_object would have similar DefaultInfo, OutputGroups code in its files and runfiles, etc.
    cgo_object = lib_result.cgo_object,
)
```