update Compat for rename of Pkg to OldPkg (#551)

* temporarily disable the Project file while things are in flux

* simply redirect Pkg to the new package manager on 0.7

Author: KristofferC

README.md

@@ -126,7 +126,9 @@ Currently, the `@compat` macro supports the following syntaxes:
   not yet part of the standard library ([#24443]).
 
 * `using Compat.Pkg` is provided on versions older than 0.7, where this library is
-  not yet part of the standard library ([#25705]).
+  not yet part of the standard library ([#25705]). Note that `Compat.Pkg` will point to
+  the new package manager on 0.7 which does not have a fully compatible API with the old
+  package manager.
 
 * `using Compat.InteractiveUtils` is provided on versions older than 0.7, where this library is
   not yet part of the standard library ([#25780]).