linkage: load formula from rack

This is to avoid TapFormulaAmbiguityError.
We should load formula based on what we installed,
rather than just name.

Library/Homebrew/dev-cmd/linkage.rb

@@ -67,7 +67,7 @@ module Homebrew
       end
 
       begin
-        f = Formula[keg.name]
+        f = Formulary.from_rack(keg.rack)
         @undeclared_deps = @brewed_dylibs.keys - f.deps.map(&:name)
         @undeclared_deps -= [f.name]
       rescue FormulaUnavailableError
@@ -122,4 +122,4 @@ module Homebrew
       end
     end
   end
-end
+end