Add `upstream` and `downstream` labels

[jaimergp]

What is the idea?

Have a label to mark issues or PRs as something that needs to be tackled by doing work in an upstream dependency.

Why is this needed?

Some issues at conda-libmamba-solver are not about our integration layer, but something that we need to do in libmamba itself or libsolv.

What should happen?

Add the following labels initially:

• upstream
• downstream
• downstream::anaconda
• downstream::conda-forge

Additional Context

~I am not sure which prefix:: it should go in 😬 ~

[kenodegard]

Could also add complimentary downstream labels:

• downstream - some downstream project or team is impacted
• downstream::anaconda - indicates that the issue impacts Anaconda's packaging teams
• downstream::conda-forge - indicates that the issue impacts conda-forge

Additional labels that could be easily added in the future:

• downstream::mamba - indicates that the issue impacts mamba
• downstream::conda-libmamba-solver - indicates that the issue impacts conda-libmamba-solver

[dkomisar]

The packaging team definitely wants a way to note which issues impact them, so this would be great.

[kalawac]

@kenodegard re: the question of what prefix these should go in, could Modifiers work?

[kenodegard]

@kalawac Ah, I see now that we've been using inconsistent terminology here, sorry for the confusion. Clarifying with terminology used in HWUG.md.

We want to define two new high-level category labels, upstream and downstream (I've provided some simple descriptions here, feel free to wordsmith further):

• upstream: this issue/PR is caused by an upstream dependency
• downstream: this issue/PR is impacting a downstream dependency

We also want to offer two additional sub-categories/topics for the downstream label:

• downstream::anaconda: this issue/PR is impacting Anaconda's packaging teams
• downstream::conda-forge: this issue/PR is impacting the conda-forge community

All of these new labels are new types, so we should pick a neutral, unused color for these labels and group them under a new header in global.yml. This header isn't too important as it's purely a comment within global.yml and just helps us keep things organized. I proposed elsewhere to call it "Upstream & Downstream" and Jannis proposed to call them "Relations", either work, so feel free to take your pick.