snapcraft's python plugins, road ahead and changes.

[Sergio Schvezov]

Hi there, after seeing many problems with the current python plugins I 
decided to overhaul them. It has basically been re-written, fixing many 
bugs and annoyances. Among those nice things these changes provide are: 
smaller resulting snaps, easier iteration, wheel support and a clear 
separation of packages brought in through pip and those provided by a 
`stage-packages` entry.

I invite any interested party to have a look at it here:

https://github.com/snapcore/snapcraft/pull/771

It is in my interest to make this part of 2.17 (which will start its SRU 
process next week).

With all this glory comes a problem, given the separation of THE python 
runtime and its stage-packages from whatever else you install iif you 
are a heavy fileset user or use command paths to entry points created 
from an install you will run into problems (a simple edit solves this).

That said I am keen to hear from people using the python plugins, and to 
get a response from them if this is a better take on it.

I am also interested to know if aside from fileset (these keywords in 
parts: `filesets`, `organize`, `stage` and `snap`) and `command` 
adaptations to whatever new path is used if there are any other blockers 
you might see with this branch.

If the previous incantations of the python plugins did not work for you 
and this one doesn't either, then please also speak up (in the PR would 
be better) and maybe we can craft a fix.

Cheers

Sergio