Fetch all remote manifests ahead of time when adding packages

[philpax]

At present, manifests (and other files, like the build metadata) are fetched on-demand by the semantic system during addition. Unfortunately, as packages are added one-by-one, this means that the fetching of the manifests is inherently serial, even when they don't depend on each other.

This can be resolved by adding an initial gather phase, in which all of the manifests are fetched in parallel, and making the rest of the semantic system purely synchronous (as it will not need to fetch anything once the initial fetches are complete).

This doesn't have much of an impact right now, but could be a problem as wider and deeper package hierarchies are created with remote-only dependencies.