Fix unused import warning on Unix systems

The NimbleError exception from the version module is being used only on
Windows platforms in the packageinstaller.nim file. Conditional import
of the module is used to fix the warning.

Related to #680

src/nimblepkg/packageinstaller.nim

@@ -3,7 +3,10 @@
 import os, strutils, sets, json
 
 # Local imports
-import version, cli, options, tools
+import cli, options, tools
+
+when defined(windows):
+  import version
 
 when not declared(initHashSet) or not declared(toHashSet):
   import common