proto: Pass in the plugin as tools to preserve runfiles (#4514)

**What type of PR is this?**

Bug fix

**What does this PR do? Why is it needed?**

The proto plugin should preserve runfiles so that bazel targets that
rely on external data can find them. It does this by by running the
plugin as a tool, which is correct because plugins are expected to be
built and run using the exec configuration.

**Which issues(s) does this PR fix?**

Fixes #4513

proto/compiler.bzl

diff --git a/proto/compiler.bzl b/proto/compiler.bzl
index ffce742..f77ab5a 100644
--- a/proto/compiler.bzl
+++ b/proto/compiler.bzl
@@ -94,8 +94,8 @@
         "valid_archive": """A Boolean indicating whether the .go files produced
 by this compiler are buildable on their own. Compilers that just add methods
 to structs produced by other compilers will set this to False.""",
-        "always_generates": """A Boolean indicating whether this compiler 
-        always generates files, regardless of whether the proto files have 
+        "always_generates": """A Boolean indicating whether this compiler
+        always generates files, regardless of whether the proto files have
         relevant definitions (e.g., services for grpc_gateway). This allows
         more strict check of compiler output.""",
         "internal": "Opaque value containing data used by compile.",
@@ -153,7 +153,7 @@
     args.add("-protoc", compiler.internal.protoc.executable)
     args.add("-importpath", importpath)
     args.add("-out_path", outpath)
-    args.add("-plugin", compiler.internal.plugin)
+    args.add("-plugin", compiler.internal.plugin.executable)
     if compiler.always_generates:
         args.add("-strict")
 
@@ -170,7 +170,6 @@
         inputs = depset(
             direct = [
                 compiler.internal.go_protoc,
-                compiler.internal.plugin,
             ],
             transitive = [transitive_descriptor_sets],
         ),
@@ -179,7 +178,7 @@
         mnemonic = "GoProtocGen",
         executable = compiler.internal.go_protoc,
         toolchain = GO_TOOLCHAIN_LABEL,
-        tools = [compiler.internal.protoc],
+        tools = [compiler.internal.protoc, compiler.internal.plugin],
         arguments = [args],
         env = go.env,
         # We may need the shell environment (potentially augmented with --action_env)
@@ -236,7 +235,7 @@
                 suffixes = ctx.attr.suffixes,
                 protoc = proto_toolchain.proto_compiler,
                 go_protoc = ctx.executable._go_protoc,
-                plugin = ctx.executable.plugin,
+                plugin = ctx.attr.plugin[DefaultInfo].files_to_run,
                 import_path_option = ctx.attr.import_path_option,
             ),
         ),