Separate building play_routes to a helper to allow remote builds

[vmax]

Fix #38
Two actions (executing routes compiler and then combining them with zipper) are collapsed into one, which allows for remote building

[pauldraper]

Is this because remote builds can't work with directories...?

Honestly, the whole zip/unzip stuff is really a pain. It would be nice if rules_scala and everything else supported directories.

[vmax]

@pauldraper yeah, the relevant issue (of not supporting directories in remote builds) is bazelbuild/bazel#6393

[pauldraper]

@vmax I thought that issue was simply that declare_directory didn't create the directory for remote builds (unlike local builds).

And that tool could just create the dir themselves.

[SrodriguezO]

@vmax have you verified that this change fixes the issue you observed in #38?

[vmax]

@SrodriguezO yes, we at @graknlabs are using our forked version extensively for ~2 weeks

[SrodriguezO]

@vmax thanks for the update. We'll merge this :)