[Fix #633, #659] Refactor `cider-find-` and `cider-jump-`.

The functions for resolving and jumping to vars and files had become
unwieldy. This makes them tractable, and uses the simplified behavior to fix:

1. The `cider-jump` and `cider-test-clear-highlights` cases where a var exists
after the file in which it was defined has been deleted.

2. The `cider-test-clear-highlights` case where the user closes one REPL session
and starts another in a different project.