Add an API for discovering system package dependencies for toolchains

[cmcgee1024]

Is your feature request related to a problem? Please describe

When I extract a Linux toolchain tarball, it has certain system package dependencies that I must know to install. The official docker images are often a good reference for what needs to be done. Swiftly also has its own lists of the packages based on the reported id and version_id in the /etc/os-release file.

It would be much better if swift.org had an API where a client like Swiftly, or other automation, can query for the system packages that are needed for the current platform.

Describe the solution you'd like

There would be some affordances in the swift.org API for listing not only the different Linux platforms that a release of the swift toolchain can support, but also the system packages for each platform needed for that toolchain.

That API can also be used to assemble download instructions for each Linux platform download.

Describe alternatives you've considered

Swiftly used to parse the docker files for the official swift docker images, but that isn't a maintainable solution. Certain changes to those docker files could break swiftly, such as single/multiple lines for the command.

Currently, swiftly maintains its own lists, which means that we have to release a new version of swiftly to contain updates to the lists over time.