Remove linux distribution-specific assumptions from swiftly

[cmcgee1024]

Swiftly in conjunction with swift.org API's (to be created) should be capable of running on virtually any Linux system, so that supported distributions can be added/removed at will without requiring swiftly updates.

This includes discovering the required system packages for a particular toolchain when installing it. Note that in this case the list can depend on the version of the toolchain being installed as system dependencies can be added and removed over time.