Adjust Source / SourceFetcher API contract

As discussed in these comments:

• #612 (comment 97721977)
• #612 (comment 97813392)

And later on IRC, we need to make a subtle change the Source / SourceFetcher API contract to ensure that all user provided configuration data will be consumed at Plugin.configure() time so that it can be appropriately validated early on, before encountering errors in mid-flight.

We agreed that:

• The documentation will assert that a Source plugin must call either Source.translate_url() or Source.mark_download_url() at Plugin.configure() time, for every URL that the user has provided in the configuration
• It will be documented that the order is significant
  • The first call to Source.mark_download_url() during Plugin.configure() will be the one used to derive the primary alias, which will be employed at Source.track() time.
• It will be asserted later on, after instantiating each SourceFetcher() but before fetching
  • We will raise an assertion if a URL marked by the SourceFetcher() has an alias, and has never been seen during Plugin.configure(), this will issue a BUG causing, the plugin author to fix their code.