It's too hard to do this correctly, and generally other approaches are better.

Add more docs around capability providers and diagnostics

microsoft#376)

* Updating CpsExtension sample to Dev 17, extending existing rule, various fixes

* Adding a few additional diagnostic environment variables and some comments

* Documentation improvements, better formatting

* Re-introducing VsSDK.targets import as .vsix stopped being produced

Removes out outdated warning. Updates code samples to use more modern syntax. Adds an overload for reading from an IVsHierarchy. Makes the section on testing project types more generic.

Update docs on finding CPS in a VS project

In a chat with a partner team today I was looking for a doc to explain
how to pull a MEF component from a project, when not a suitably-scoped
MEF component yourself.

This information is touched on in `finding_CPS_in_a_VS_project.md`, but
feels more discoverable in `mef.md`.

…onents-from-projects

Document obtaining MEF components from projects

…bilites-from-dtb

Doc that capabilities cannot come from DTB