display exports for local packages

Display exports for local packages in ‘meteor show’. (We will eventually display exports for non-local packages as well)

- method to aggregate exports for a package in packageSource (exports are per-architecture).

- get this data from packageSource in PackageQuery for ‘meteor show’. Don’t store it in the
  local catalog — while it is not a particularly expensive operation, it is still more expensive
  than a simple lookup. We really do care about minimizing any sort of computation when we
  are initializing packages, since we want the tool to be fast.

- display the data in ‘meteor show’. It makes sense to line wrap this with the ‘Exports:’ label as a
  bulletPoint (just look at the test to see an example where this improves user experience). Since we
  are doing that, we might as well use that bulletPoint functionality on the other labels as well.

There is also a test. Run ‘meteor self-test show’ to test, or run ‘meteor show’ on a local package
with exports.

The Troposphere counterpoint to this is: https://github.com/meteor/troposphere/pull/5
Maybe avoid calling variables `exports`, since that looks scarily like the Node exports thing?