std/private/paths: hardcoded paths reusable amongst stdlib, compiler, tools etc

## proposal
* add `std/private/paths` containing constant paths already used in compiler, stdlib, tools, to avoid duplication and make it easier to document

## links
* testament/lib/stdtest/specialpaths.nim
* see also hardcoded paths mentioned in https://github.com/nim-lang/Nim/pull/17132

