Make the NNBD fork not stomp on the other build.

Add a new top level target to build the NNBD fork of the SDK. In its
BUILD.gn file, update all of the paths to not step on the main dart-sdk
directory.

This probably isn't perfect, but it's a first step.

Change-Id: I7ac12a2b174faf6b19d88fff54e3438299577a26
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/117681
Reviewed-by: Nicholas Shahan <nshahan@google.com>
Commit-Queue: Bob Nystrom <rnystrom@google.com>

BUILD.gn

@@ -86,6 +86,12 @@ group("create_sdk") {
   ]
 }
 
+group("create_sdk_nnbd") {
+  public_deps = [
+    "sdk_nnbd:create_sdk_nnbd",
+  ]
+}
+
 if (defined(dart_host_sdk_toolchain) &&
     dart_host_sdk_toolchain != host_toolchain) {
   group("create_host_sdk") {