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

tests/modules/mforwarded_pure_enum.nim

@@ -0,0 +1,3 @@
+
+import mforwarded_pure_enum2
+export mforwarded_pure_enum2.PureEnum

tests/modules/mforwarded_pure_enum2.nim

@@ -0,0 +1,4 @@
+
+type
+  PureEnum* {.pure.} = enum
+    x, y, z

tests/modules/tfowarded_pure_enum.nim

@@ -0,0 +1,7 @@
+discard """
+  output: '''z'''
+"""
+
+import mforwarded_pure_enum as t2
+
+echo z