Add back Truffle::MAIN as it is used by interop code

* Move it closer to its only usage and comment about it.

Author: eregon

src/main/ruby/core/truffle/interop.rb

@@ -8,8 +8,10 @@
 # GNU General Public License version 2, or
 # GNU Lesser General Public License version 2.1.
 
-module Truffle
+# Used by Truffle::Interop.lookup_symbol
+Truffle::Interop::MAIN = self
 
+module Truffle
   module Interop
 
     def self.import_method(name)