make repls for all targets

now that bazel run can support repls without any silly dance

see: https://groups.google.com/d/msg/bazel-discuss/hXpklreOPik/2Suxd84dAgAJ

maybe we should make a `.repl` runnable target as an optional output of all rules. Then users can do
```
bazel run --direct_run //foo/bar:baz.repl
```
and be dropped directly into a repl session with the same classpath as the target.
I think this would be a nice usability win to avoid having to manually make a second target, and if it is an optional output it shouldn't slow down the build unless needed.

What do you think @ittaiz

Start by reviewing how the repository's rules expose runnable targets and how `bazel run --direct_run` handles the proposed `.repl` target. Done means an optional `.repl` runnable target is available for all relevant rules, opens a REPL with the target's classpath, and does not add build work unless requested.

Written by the indexing model from the issue text.