Faster package querying

[and3rson]

During the docker-py distribution check, pip freeze can be terribly slow if you have a lot of packages installed. Here's my case:

$ time pip freeze > /dev/null
real    0m3,930s
user    0m3,787s
 sys     0m0,093s

$ pip freeze | wc -l
173

$ time docker-compose 2> /dev/null
real    0m3,443s
user    0m3,277s
sys     0m0,143s

After this fix it becomes more than 3x faster:

$ time docker-compose 2> /dev/null 
real    0m0,935s
user    0m0,827s
sys     0m0,100s

This feature was tested in Python 2.7 & 3.6.

[GordonTheTurtle]

Please sign your commits following these rules:
https://github.com/moby/moby/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "feature/faster-package-querying" git@github.com:and3rson/compose.git somewhere
$ cd somewhere
$ git rebase -i HEAD~842354498736
editor opens
change each 'pick' to 'edit'
save the file and quit
$ git commit --amend -s --no-edit
$ git rebase --continue # and repeat the amend for each commit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.

[and3rson]

The Jenkins build seems to be hanging... Is it normal?

[shin-]

#4920 (comment)

[shin-]

This part of the code has been iterated over many times. The current solution is the one that offers the best compromise for every declination of the software.