Support downloading microchip packs

[kiram9]

Microchip publishes packs at https://packs.download.microchip.com/ It would be awesome to support these packs, or other custom urls.

[flit]

Strange, It only takes an email to the CMSIS/Keil team to add CMSIS-Packs to the main registry, so I don't know why Microchip wouldn't do it. Although, they include packs for non-Arm devices, and their packs have a non-standard .atpack extension.

Anyway, the current recommendation is to download the pack you need from the Microchip site and reference it locally using the --pack option.

I'll consider adding support for custom registry URLs, but it will be low priority.