Plugins globbing for files can be slow

[nathankleyn]

Plugins currently register their intent to hook into a project through the use of globs:

class TestPlugin < Plugin
  projects '**/Gemfile'
end

However, this causes every plugin to traverse the directory tree of the Shanty project over and over again, leading to things being quite slow.

Plugins should share a pre-calculated directory tree which they can glob, avoiding disk access.