Avoid excessive BES event size (#79)

The files in requested output groups are announced as events on the
Build Event Stream (BES), with declared directories expanded into the
list of files they contain. Since mypy caches contain information about
all transitive dependencies, including the cache directory in the `mypy`
output group thus resulted in a very large number of (duplicated) files
announced in build events when the aspect is applied to an entire repo,
causing excessive upload times and BES sizes in the GBs.

This is fixed by dropping the cache directory from the `mypy` output
group. It is still accessible via the `MypyCacheInfo` provider.

Author: fmeum

mypy/private/mypy.bzl

@@ -151,18 +151,15 @@ def _mypy_impl(target, ctx):
     args = ctx.actions.args()
     args.add("--output", output_file)
 
+    result_info = [OutputGroupInfo(mypy = depset([output_file]))]
     if ctx.attr.cache:
         cache_directory = ctx.actions.declare_directory(ctx.rule.attr.name + ".mypy_cache")
         args.add("--cache-dir", cache_directory.path)
 
         outputs = [output_file, cache_directory]
-        result_info = [
-            MypyCacheInfo(directory = cache_directory),
-            OutputGroupInfo(mypy = depset(outputs)),
-        ]
+        result_info.append(MypyCacheInfo(directory = cache_directory))
     else:
         outputs = [output_file]
-        result_info = [OutputGroupInfo(mypy = depset(outputs))]
 
     args.add_all([c.path for c in upstream_caches], before_each = "--upstream-cache")
     args.add_all([s for s in ctx.rule.files.srcs if "/_virtual_imports/" not in s.short_path])