add -d:nimStrictMode in CI to keep code from regressing; fixes ConvFromXtoItselfNotNeeded, UnusedImport notes (#16764)

compiler/condsyms.nim

@@ -126,4 +126,5 @@ proc initDefines*(symbols: StringTableRef) =
   defineSymbol("nimHasCastPragmaBlocks")
   defineSymbol("nimHasDeclaredLocs")
   defineSymbol("nimHasJsBigIntBackend")
+  defineSymbol("nimHasWarningAsError")
   defineSymbol("nimHasHintAsError")

compiler/dfa.nim

@@ -32,8 +32,6 @@
 import ast, types, intsets, lineinfos, renderer
 import std/private/asciitables
 
-from patterns import sameTrees
-
 type
   InstrKind* = enum
     goto, fork, def, use