mpm collections: allow removing things from path

[mobeets]

Let's say you have a collection called test, and you 'init' it:

>> mpm init -c test
Using collection "test"
   Adding to path: /Users/mobeets/code/mpm/mpm-packages/mpm-collections/test/cbrewer
   Added paths for 1 package(s).

If collections are meant to be self-contained (like virtual environments), then before init-ing, we should removing any paths added from the previous collection (e.g., the default one). This would require:

• saving state about which collection is currently init'd
• when a new collection is init'd: 1) rm from the path any packages that were part of the previous collection, and then 2) update the state about which collection is currently init'd

We should probably also add a flag that says to not rm paths. For example, if you want to be able to init multiple collections in series.