[FR]: Support for "virtual" dependencies

[alexeagle]

What is the current behavior?

It's difficult to work in a Python monorepo with a single set of requirements, because not all teams are using aligned dependency versions today. However, multiple requirements are difficult because a transitive dependency of an application can use a conflicting version from a direct dependency.

Describe the feature

References:

• background reading: https://blog.aspect.dev/multiple-deps
• design doc from Aspect: https://docs.google.com/document/d/1A1ZGSLn7WTbWdBVxNQtB9c3fZTFITprdwhru89_O28A/edit#heading=h.i23ncs9b913x
• specification from Verkada: https://docs.google.com/document/d/1yLNMt3rRbvK80evKOIpqxmwzuMP7THh371L85QkwMx4/edit
  A PR with one implementation: Demonstrate virtual dependencies #87