internal/fieldnum: generate field numbers for the google.protobuf package

Generate field numbers for the well-known types,
so that encoding/jsonpb can benefit from them as well.

This CL fixes internal/cmd/generate-protos, which was silently failing
because the modulePath was not properly initialized. We fix this by
moving it to the start of the init function.

Change-Id: I87637176f29218cffa512b4baa49f39dae924061
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/168497
Reviewed-by: Herbie Ong <herbie@google.com>
diff --git a/internal/fieldnum/wrappers_gen.go b/internal/fieldnum/wrappers_gen.go
new file mode 100644
index 0000000..6882713
--- /dev/null
+++ b/internal/fieldnum/wrappers_gen.go
@@ -0,0 +1,52 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style.
+// license that can be found in the LICENSE file.
+
+// Code generated by generate-protos. DO NOT EDIT.
+
+package fieldnum
+
+// Field numbers for google.protobuf.DoubleValue.
+const (
+	DoubleValue_Value = 1 // optional double
+)
+
+// Field numbers for google.protobuf.FloatValue.
+const (
+	FloatValue_Value = 1 // optional float
+)
+
+// Field numbers for google.protobuf.Int64Value.
+const (
+	Int64Value_Value = 1 // optional int64
+)
+
+// Field numbers for google.protobuf.UInt64Value.
+const (
+	UInt64Value_Value = 1 // optional uint64
+)
+
+// Field numbers for google.protobuf.Int32Value.
+const (
+	Int32Value_Value = 1 // optional int32
+)
+
+// Field numbers for google.protobuf.UInt32Value.
+const (
+	UInt32Value_Value = 1 // optional uint32
+)
+
+// Field numbers for google.protobuf.BoolValue.
+const (
+	BoolValue_Value = 1 // optional bool
+)
+
+// Field numbers for google.protobuf.StringValue.
+const (
+	StringValue_Value = 1 // optional string
+)
+
+// Field numbers for google.protobuf.BytesValue.
+const (
+	BytesValue_Value = 1 // optional bytes
+)