Handle targets provided in deps as a result of GoSource.resolver (#4063)

**What type of PR is this?**
Starlark fixup

**What does this PR do? Why is it needed?**
Make the migration process a little smoother

**Which issues(s) does this PR fix?**
#4030 (comment)

**Other notes for review**

go/private/context.bzl

@@ -259,7 +259,7 @@ def _dedup_archives(archives):
         deduped_archives.append(arc)
     return deduped_archives
 
-def _library_to_source(go, attr, library, coverage_instrumented):
+def _library_to_source(go, attr, library, coverage_instrumented, verify_resolver_deps = True):
     #TODO: stop collapsing a depset in this line...
     attr_srcs = [f for t in getattr(attr, "srcs", []) for f in as_iterable(t.files)]
     generated_srcs = getattr(library, "srcs", [])
@@ -311,6 +311,19 @@ def _library_to_source(go, attr, library, coverage_instrumented):
     if library.resolve:
         library.resolve(go, attr, source, _merge_embed)
 
+        # TODO(zbarsky): Remove this once downstream has a chance to migrate.
+        if verify_resolver_deps:
+            has_targets = False
+            for dep in source["deps"]:
+                if type(dep) == "Target":
+                    has_targets = True
+                    break
+            if has_targets:
+                print('Detected Targets in `source["deps"]` as a result of _resolver. ' +
+                      "Please pass a list of `GoArchive`s instead, for examples `deps = [deps[GoArchive] for dep in deps]`. " +
+                      "This will be an error in the future.")
+                source["deps"] = [get_archive(dep) for dep in source["deps"]]
+
     return GoSource(**source)
 
 def _collect_runfiles(go, data, deps):

proto/def.bzl

@@ -146,7 +146,7 @@ def _go_proto_library_impl(ctx):
         resolver = _proto_library_to_source,
         srcs = go_srcs,
     )
-    source = go.library_to_source(go, ctx.attr, library, False)
+    source = go.library_to_source(go, ctx.attr, library, False, verify_resolver_deps = False)
     providers = [library, source]
     output_groups = {
         "go_generated_srcs": go_srcs,