support user-defined plugins

[oconnor663]

Maybe support a plugins: field in peru.yaml, given as a path or a list of paths.

[oconnor663]

Ideally we'd like to be able to let the user import plugin definitions. (Like say, cloning down a github repository that defines a new peru plugin.) That will be tricky, because right now we construct import trees all at once, and the imports that use this custom plugin type will be errors ("plugin foo not found") before the imports that define the plugin are visible.

Doing imports in multiple stages sounds hacky, but it could work. We could also unpack plugins "in space" in a special step before we run the real fetch. That would have the major downside of needing to unpack the plugin every time a fetch is done. Maybe the .peru dir could have a special spot for unpacked plugins, to keep them around?

NOTE: This is similar to the problem that comes up when we think about "environment variable imports." The issue there is that the tmp dir containing the imported files needs to be constructed every time a build is run, because there's no guarantee that the last build didn't modify the files. That feels scandalously inefficient. Is there a way to have peru create a directory that's impossible to modify?

[oconnor663]

To clarify, that would be importing new plugins inside the current project, not installing them somewhere permanent on the user's system.

[oconnor663]

https://phabricator.buildinspace.com/D27 adds the plugins: field, but we're punting on any kind of special fetching. Peru is sometimes maybe better than copy-paste, but not necessarily this time.

[oconnor663]

275e8fc