merge workspace, virtual project, and dependencies into just dependencies

[majidaldo]

Functionally, I don't see why there needs to be a distinction between them. What if I want to treat a "workspace" as a dev "dep"? (i don't want it to be a "virtual" package)

The user should not have to worry about the distinctions. The project manager should figure out which one represents some grouping of dependencies to pass on to the installer.

You could also roll the notion of feature flags into this system. See processing selectors in conda build.