Generated code should use a string uri to point at the library it is a part of

[jakemac53]

Today generated json_serializable code triggers the use_string_in_part_of_directives lint - we should really not be doing this by name but by uri.

[jakemac53]

(this might actually be a source_gen issue, haven't looked into it)

[kevmoo]

It should follow the pattern used by the part usage from the source library IIRC.

[kevmoo]

Pretty sure this is a source_gen issue, @jakemac53 !

[jakemac53]

see dart-lang/source_gen#665