Make `pow dist load` that unpacks a dist created by `pow dist save`

[mwatts15]

Loading a dist should allow to use the data exactly as it was packaged up to a change in environment-specific variables like proxy URLs, access credentials, etc.

Importing contexts from outside presents some difficulties for graph storage given the existing storage scheme. In particular, a given context identifier can no longer masquerade as a key for the actual content in a context since different sources can state different things in the same context (setting aside whether the two groups of statements are logically consistent with each other). So, it would be necessary to introduce a key for each graph

[mwatts15]

The notion of a dist has been subsumed by bundles, and we have bundle install and fetch implemented, so this is sorted.