Make find-definition work on code eval'd via the repl.

[austinhaas]

When code is originally loaded, via require, symbols will have metadata associated with them that indicates the file and line from where it was loaded. If you eval the code after that, :file will be changed to "NO_SOURCE_PATH".

I really don't know the best way to handle this. That metadata really only tells you where the code originally came from; if you edit any of the source files, that info may become stale.

It would be good to know how Clojure sets those values in the first place. This might be a good place to start: clojure/clojure@6bbfd94