[RFC] add 'fetch exe' and 'reup exe' to plugin.yaml

Summary:
Previously we always used `fetch.*` and `reup.*` paths to invoke
plugins. That any-extension bit on the end was pretty messy, and it also
forced plugins supporting reup to have at least 2 script files (in
practice, at least 3). With this diff, `plugin.yaml` now requires the
`fetch exe` field, which defines the path that gets invoked. Plugins
that support reup will also define `reup exe`.

Those two exes can be the same script, in which case the script will
need to switch on the `PERU_PLUGIN_COMMAND` variable to figure out
whether it's doing a fetch or a reup. I actually prefer that overhead to
splitting up everything in different files, but I'd appreciate comments
on this. This diff only updates our plugins' `plugin.yaml` files; my
next diff will reorganize plugins into single scripts.

See https://github.com/buildinspace/peru/issues/45

Reviewers: sean

Differential Revision: https://phabricator.buildinspace.com/D97
[RFC] reorganize plugins into unified scripts

Summary:
Now that plugin.yaml has the 'fetch exe' and 'reup exe' fields (and now
that we define the PERU_PLUGIN_COMMAND variable), we can reorganize all
of our plugins into single files. Since they're all fairly small
scripts, I prefer that to creating *_shared.py everywhere. The plugins
that were already one file just get some more descriptive names, like
`cp_plugin.py` instead of `fetch.py`.

I think I like the results of this diff, but I want to hear what you
think, @sean.

https://github.com/buildinspace/peru/issues/45

Reviewers: sean

Subscribers: sean

Differential Revision: https://phabricator.buildinspace.com/D98