Checking if extension is already installed/loaded

[adhocore]

Right now if we (re)run the command below,

pickle install apcu

It starts downloading and installing the apcu ext right away without checking if it is already there.

While it is upto user to know which exts are already there and not try to install them over again, it maybe useful for pickle to do this in itself.

Alternatively, there may be --reinstall flag which will work as current behavior.

[pierrejoye]

--reinstall could be an option however it won't work if the php used by pickle is the one using the extension being installed (locked)