makes DuplicateModuleImport back to an error (nim-lang#25178)

fixes nim-lang#24998

Basically it retraces back to the situation before
nim-lang#18366 and
nim-lang#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 nim-lang#18762
nim-lang#20907

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

Author: ringabout

changelog.md

@@ -25,6 +25,8 @@ errors.
 
 - `std/parsesql` has been moved to a nimble package, use `nimble` or `atlas` to install it.
 
+- With `-d:nimPreviewDuplicateModuleError`, importing two modules that share the same name becomes a compile-time error. This includes importing the same module more than once. Use `import foo as foo1` (or other aliases) to avoid collisions.
+
 ## Standard library additions and changes
 
 [//]: # "Additions:"

compiler/condsyms.nim

@@ -173,4 +173,5 @@ proc initDefines*(symbols: StringTableRef) =
   defineSymbol("nimHasXorSet")
 
   defineSymbol("nimHasSetLengthSeqUninitMagic")
+  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

lib/pure/terminal.nim

@@ -100,7 +100,7 @@ const
   stylePrefix = "\e["
 
 when defined(windows):
-  import std/[winlean, os]
+  import std/os
 
   const
     DUPLICATE_SAME_ACCESS = 2
@@ -926,8 +926,6 @@ when defined(windows):
     stdout.write "\n"
 
 else:
-  import std/termios
-
   proc readPasswordFromStdin*(prompt: string, password: var string):
                             bool {.tags: [ReadIOEffect, WriteIOEffect].} =
     password.setLen(0)
@@ -981,9 +979,6 @@ proc isTrueColorSupported*(): bool =
   ## Returns true if a terminal supports true color.
   return getTerminal().trueColorIsSupported
 
-when defined(windows):
-  import std/os
-
 proc enableTrueColors*() =
   ## Enables true color.
   var term = getTerminal()

lib/system/alloc.nim

@@ -12,7 +12,6 @@
 
 include osalloc
 import std/private/syslocks
-import std/sysatomics
 
 template track(op, address, size) =
   when defined(memTracker):

lib/system/strmantle.nim

@@ -8,9 +8,7 @@
 #
 
 # Compilerprocs for strings that do not depend on the string implementation.
-
-import std/private/digitsutils
-
+import std/private/digitsutils as digitsutils2
 
 proc cmpStrings(a, b: string): int {.inline, compilerproc.} =
   let alen = a.len