ensime support, other emacs support

[cosmicexplorer]

See #2939. There are multiple existing pants .ensime file generator plugins, but they are in varying degrees of updated-ness, both with versions of pants and of ensime. I took a stab at doing this myself using the export task and a jvm tool from scala sources which builds against ensime-api to produce a proper .ensime file -- this is the EnsimeGen task in this pants-plugins/ directory (there's lots of other stuff too, sorry).

This plugin works very reliably for me, with some remaining issues detailed in this comment, which is why I'd like to upstream it somewhere (and it seems from the previous ticket that should probably be in the ensime project?). The only reason it would be specifically this plugin I wrote is that (1) it uses the export task, without any private APIs (2) I haven't been able to get any of the others to work. But I would love to do this work upstream so the previous work in this area isn't erased just because I couldn't figure it out.

One other thing that was mentioned in the previous ticket was a potential pants-mode for emacs -- I am not as familiar with sbt-mode as I would like because I've been using pants so much, but I would love to discuss what such a package could be useful for. I have published and maintain several elisp packages on MELPA, many of which are visible on my github profile. Since emacs + pants is 100% of my daily workflow, this is something I'm willing to invest some time into if we can discuss how to do that most effectively.

[alamaison]

@cosmicexplorer In #2939 you say "It ends up building a scala source which consumes the ensime-api jar to produce compatible .ensime files when invoked as a jvm tool, with ./pants ensime <targets...>". Could you explain what you mean for a PANTS/Scala n00b?

[alamaison]

And by what do you mean I mean how do I install it as a pants goal? And actually how do I build it too?