faster determinePlatform

- 310ms -> 7ms

- don't link dub_platform_probe

- avoid phobos imports in the probe file
store metadata cache on disk

- Writing is a little slow due to Json escaping
  so also use a dirty flag to track when a writeback
  is needed.

- The metadata contains readme files for every project,
  strip that data for now because of the point above.
create symlinks instead of copying cached build files
Merge pull request #388 from MartinNowak/fasterDub

Make DUB rebuild faster.