Ruby: Remove CFG dependency from AST stage

Commit 028ef6f27f had the unintended side-effect
that the AST and CFG stages got merged, because the AST stage's `isCapturedAccess`
now depends on `getCfgScopeImpl`, which belongs to the CFG stage.

The fix is to remove `getCfgScopeImpl` from the CFG stage, and instead let it
be part of the AST stage.

ruby/ql/lib/codeql/ruby/controlflow/internal/ControlFlowGraphImpl.qll

@@ -1041,6 +1041,9 @@ private Scope parent(Scope n) {
   not n instanceof CfgScope::Range_
 }
 
+cached
+private CfgScope getCfgScopeImpl(AstNode n) { result = parent*(scopeOfInclSynth(n)) }
+
 /** Gets the CFG scope of node `n`. */
 pragma[inline]
 CfgScope getCfgScope(AstNode n) {
@@ -1052,13 +1055,6 @@ CfgScope getCfgScope(AstNode n) {
 
 cached
 private module Cached {
-  /** Gets the CFG scope of node `n`. */
-  cached
-  CfgScope getCfgScopeImpl(AstNode n) {
-    forceCachingInSameStage() and
-    result = parent*(scopeOfInclSynth(n))
-  }
-
   cached
   newtype TSuccessorType =
     TSuccessorSuccessor() or