shift code from DirectoryImporter out to a common area.

remove use of "os" module (bootstrap issues) and go to the underlying
  platform-specific modules
fix problem in _compile() (trapped wrong error on permission issues)
add SysPathImporter and BuiltinImporter
put __file__ into modules imported from the filesystem. [backwards compat]
put __path__ into modules [backwards compat]
  oops: it is doing this for all modules, not just packages.
comment and tweak to the PackageArchiveImporter