Scope chaining projects

[psakievich]

Projects should be able to be chained from one to another.

Project_a should be able to have a context that depends on a context from Project_b.

Examples:

• Application A has a project with a context based on a gcc toolchain. Ideally they should be able to have that context depend on the system administrator's project, which has a context to supply the gcc toolchain
• Application A's project wants to depend on application B's so they can test they compile together.