Use platform-specific configuration directories to store swiftly files

[patrickfreed]

The design and current implementation assume that swiftly, its configuration, and its toolchains will be stored in ~/.swiftly. This ignores certain platform-specific conventions, such as the XDG base directory specification, which indicates it should be stored in ~/.local/share/swiftly. We should adopt these conventions where possible.

[tomerd]

in SwiftPM we use the platform idiomatic directories and then create symlinks from the home directory, since many users expect it there