Improving the cache of the declaration finder for external references

[bclothier]

From a discussion in chat, there is one spot where we could improve the cache. Currently when a project has more reference, it becomes more and more slower to parse because of # of declarations that must be analyzed and eventually resolved from the external references.

The proposal is to cache the declarations from external references and refresh only if the user has actually changed the references (which is a comparatively rare event) and the initial parse. That would help the performance in where a project references several external libraries.

Note that implementing this will be a challenge as the collections within the finder are loaded lazily and one must be careful to ensure that cache does not become stale.