make koch and tools work with nimPreviewSlimSystem (#20459)

tools/deps.nim

@@ -1,6 +1,9 @@
-import os, uri, strformat, strutils
+import std/[os, uri, strformat, strutils]
 import std/private/gitutils
 
+when defined(nimPreviewSlimSystem):
+  import std/assertions
+
 proc exec(cmd: string) =
   echo "deps.cmd: " & cmd
   let status = execShellCmd(cmd)

tools/detect/detect.nim

@@ -12,7 +12,10 @@
 # The second one is more portable, and less semantically correct. It only works
 # when there's a backing C compiler available as well, preventing standalone
 # compilation.
-import os, strutils
+import std/[os, strutils]
+
+when defined(nimPreviewSlimSystem):
+  import std/syncio
 
 when defined(openbsd) or defined(freebsd) or defined(netbsd):
   const

tools/kochdocs.nim

@@ -1,6 +1,10 @@
 ## Part of 'koch' responsible for the documentation generation.
 
-import os, strutils, osproc, sets, pathnorm, sequtils
+import std/[os, strutils, osproc, sets, pathnorm, sequtils]
+
+when defined(nimPreviewSlimSystem):
+  import std/assertions
+
 # XXX: Remove this feature check once the csources supports it.
 when defined(nimHasCastPragmaBlocks):
   import std/pegs