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/certificates/tx.template

@@ -93,7 +93,7 @@
 	]
       , [ 2
         , [ 1
-          , h'{{ certificates.purposes_publish.hash }}'
+          , h'{{ certificates.purposes.publish.hash }}'
 	  ]
 	, h'11111111111111111111111111111111111111111111111111111111'
 	]
@@ -109,7 +109,7 @@
 
     { 5: [[2, 20, 121([]), [1000000, 100000000]]]
 
-    , 7: [h'{{ certificates.purposes_publish.cbor }}']
+    , 7: [h'{{ certificates.purposes.publish.cbor }}']
     },
 
     true,