Feature/unload user packrat

This is kind of hairy and I think it warrants a better solution -- if someone is currently using a version of packrat in the user library, we want to unload it then load the private version of packrat.

We still need to make sure that packrat has actually been installed in the private library before moving ahead here, but there is probably a better way to approach this (e.g. @jmcphers's suggestion to separate logic so that e.g. `setPackratModeOn` is structured as:

```
beforePackratModeOn(...)
setPackratModeEnvironmentVar()
afterPackratModeOn(...)
```

or something to that effect.

For now, we just copy 'mutables' from the 'user packrat namespace' to the 'private packrat namespace', if necessary.