makes DuplicateModuleImport back to an error (#25178)

fixes #24998

Basically it retraces back to the situation before
https://github.com/nim-lang/Nim/pull/18366 and
https://github.com/nim-lang/Nim/pull/18362, i.e.

```nim
import fuzz/a
import fuzz/a
```

```nim
import fuzz/a
from buzz/a
```

```nim
import fuzz/a except nil
from fuzz/a import addInt
```

All of these cases are now flagged as invalid and triggers a
redefinition error, i.e., each module name importing is treated as
consistent as the symbol definition

kinda annoying for importing/exporting with `when conditions` though

ref https://github.com/nim-lang/Nim/issues/18762
https://github.com/nim-lang/Nim/issues/20907

```nim
from std/strutils import toLower
when not defined(js):
  from std/strutils import toUpper
```

(cherry picked from commit 87ee9c84cb)

compiler/condsyms.nim

@@ -171,3 +171,5 @@ proc initDefines*(symbols: StringTableRef) =
   defineSymbol("nimHasJsNoLambdaLifting")
   defineSymbol("nimHasDefaultFloatRoundtrip")
   defineSymbol("nimHasXorSet")
+
+  defineSymbol("nimHasPreviewDuplicateModuleError")

compiler/lookups.nim

@@ -388,7 +388,7 @@ proc addDeclAt*(c: PContext; scope: PScope, sym: PSym, info: TLineInfo) =
   if sym.name.id == ord(wUnderscore): return
   let conflict = scope.addUniqueSym(sym)
   if conflict != nil:
-    if sym.kind == skModule and conflict.kind == skModule:
+    if sym.kind == skModule and conflict.kind == skModule and not c.config.isDefined("nimPreviewDuplicateModuleError"):
       # e.g.: import foo; import foo
       # xxx we could refine this by issuing a different hint for the case
       # where a duplicate import happens inside an include.

compiler/nim.cfg

@@ -12,6 +12,8 @@ define:nimPreviewNonVarDestructor
 define:nimPreviewCheckedClose
 define:nimPreviewAsmSemSymbol
 define:nimPreviewCStringComparisons
+define:nimPreviewDuplicateModuleError
+
 threads:off
 
 #import:"$projectpath/testability"

compiler/suggest.nim

@@ -35,7 +35,7 @@
 import prefixmatches, suggestsymdb
 from wordrecg import wDeprecated, wError, wAddr, wYield
 
-import std/[algorithm, sets, parseutils, tables, os]
+import std/[algorithm, sets, parseutils, os]
 
 when defined(nimsuggest):
   import pathutils # importer