Add a new `add_source` method that will add an individual `Source`
to a `Manifest` give its `ModuleInfo` and options. The dictionary
of source options in the manifest is replaced with a list of such
`Sources` and `add_source` will append to it. Adap the version 1
format code to use `add_source` and reconstruct the source options
from the list of source on `describe`.
Remove the `sources_options` constructor parameter for `Manifest`
and adapt all the source base for this.
12c9173faf