Use dot to separate purpose from validator name in blueprint

The rationale is two folds:

  1. It's more consistent with how we already separate the validator
  name from its module.

  2. Because `_` may be present in Aiken's validator's name, it is hard
     to read and I am afraid it could potentially conflict later on. So
     it's better to use a separator that cannot appear in validator
     names.

examples/acceptance_tests/script_context/v3/ctx/withdraw/tx.template

@@ -23,7 +23,7 @@
 
     , 5:
       { h'E000000000000000000000000000000000000000000000000000000000': 14
-      , h'F0{{ withdraw.thing_withdraw.hash }}': 0
+      , h'F0{{ withdraw.thing.withdraw.hash }}': 0
       }
 
     , 14: