Fine grained download control and version status

[flit]

Currently the only the entire set of PDSCs can be cached as a whole.

It would be very useful to have the abilities to separately:

• Download only the index.
• Determine if and which PDSCs and (downloaded) packs are out of date (or entirely new)
• Download/update specified PDSCs.

The primary use case is to quickly check for new PDSCs and versions of already downloaded packs, and then download only those that are needed. Specifically, it would be highly useful if pyocd could quickly check for a new PDSC/pack, and potentially download it, when it is connecting to a target.