Ensure swiftly can parse tags from apple/swift repository #69
Description
swiftly currently can parse toolchain selectors of the form a.b-DEVELOPMENT-SNAPSHOT-YYYY-mm-dd-a, but tags at https://github.com/apple/swift/tags have a swift- prefix, which swiftly cannot parse. We should update the parsing to accept that so that users can easily install a toolchain based on a git tag.