Add channel specific job with new str

[AntoinePrv]

Currently, using a spec like conda-forge::python will segfault in MSolver::all_problems_structured because channel specific jobs are added as a queue qith SOLVER_JOB_ONE_OF. In all_prblem_structured, the queue id is popping back as a dep but isn't.
There is not easy way to check for it explicitly, and no way to kepp track of the the fact that it was an explicit channel job.

The fix proposed here is rather to change the implementation of add_channel_specific_jobs to dynamically add a spec for the channel specific spec.

[AntoinePrv]

@wolfv Regarding the conda-forge::python syntax, this is mamba specific?
I wonder if an python@conda-forge would be better to work with URL channels (a separate reflection)

[wolfv]

No, it is not mamba specific. The channel:namespace:packagename syntax comes from conda https://github.com/conda/conda/blob/2e3f909cba109cdd7642387722223aa5cb79fd5c/conda/models/match_spec.py#L88-L92

[jonashaag]

fyi @ThomasBlauthQC