refactorings to prepare the compiler for IC (#15935)

* added ic specific Nim code; WIP
* make the symbol import mechanism lazy; WIP
* ensure that modules can be imported multiple times
* ambiguity checking
* handle converters and TR macros properly
* make 'enum' test category green again
* special logic for semi-pure enums
* makes nimsuggest tests green again
* fixes nimdata
* makes nimpy green again
* makes more important packages work

compiler/plugins/locals.nim

@@ -19,8 +19,7 @@ proc semLocals*(c: PContext, n: PNode): PNode =
   tupleType.n = newNodeI(nkRecList, n.info)
   let owner = getCurrOwner(c)
   # for now we skip openarrays ...
-  for scope in walkScopes(c.currentScope):
-    if scope == c.topLevelScope: break
+  for scope in localScopesFrom(c, c.currentScope):
     for it in items(scope.symbols):
       if it.kind in skLocalVars and
           it.typ.skipTypes({tyGenericInst, tyVar}).kind notin