Migrate buf config (#294)

Mostly via `buf config migrate`, but then nixing the backwards
compatible configs in favor of the defaults.

Author: stefanvanburen

buf.gen.yaml

@@ -1,9 +1,10 @@
-version: v1
+version: v2
 managed:
   enabled: true
-  go_package_prefix:
-    default: github.com/bufbuild/makego/internal/gen/proto/go
+  override:
+    - file_option: go_package_prefix
+      value: github.com/bufbuild/makego/internal/gen/proto/go
 plugins:
-  - name: go
+  - local: protoc-gen-go
     out: internal/gen/proto/go
     opt: paths=source_relative

buf.work.yaml

@@ -0,0 +1,3 @@
+version: v2
+modules:
+  - path: proto