Code review comment for ~mertkirpici/juju-lint:lp/1989575

So after giving this some thought and experimenting, I came to a conclusion that having a central dev requirements ends up installing ALL of the dependencies for every tox environment, which is very redundant and takes a lot of time and space.

Therefore I moved away from that idea towards the opposite.
With the second change I pushed, every tox env installs only their own dependencies they need. (Most charms also follow this)

Also another change is that, the virtual env creation in makefile's `dev-environment` target was not being created by tox like all the rest but it was using the package `virtualenv`. I moved away from that also, in favor of using tox for the `dev-environment` creation as well.

Here is a showcase of all current make+tox targets :)
https://pastebin.ubuntu.com/p/ZkxG8mFsWV/