RFE - List installed, third-party packages in the Explorer View (with ability to add/remove/update, etc)

_From @DonJayamanne on August 9, 2017 13:8_

- Ability to detect packages installed while vs code is running
- Ability to detect packages installed but not listed in requirements & vice versa


_Copied from original issue: DonJayamanne/pythonVSCode#1153_