[WIP] bring back std/ prefix within compiler and ensure it works in bootstrap + bsd

* refs #16282 (comment)
* sounds very similar to #14291

Author: timotheecour

compiler/ic/dce.nim

@@ -9,7 +9,7 @@
 
 ## Dead code elimination (=DCE) for IC.
 
-import intsets, tables
+import std/[intsets, tables]
 import ".." / [ast, options, lineinfos, types]
 
 import packed_ast, ic, bitabs