RFC: Support "foreign" package providers

[mwoehlke]

In order to ease adoption, it would be helpful if CPS provided a way to specify dependencies on things which are not described by CPS. In particular, pkg-config is a likely target for such a feature.

I propose adding a provider attribute to <requirement> objects, with supported values cps and pkgconf.

A value of pkgconf shall inform tools to search for <name>.pc, which shall provide the component(s) <name>:shared and/or <name>:static. The tool shall respect the version and components attributes in the usual fashion.

[mwoehlke]

My original thought was to actually resolve libraries rather than just provide "flag soup", which would have made :static/:shared feasible.

After a lengthy, private discussion in which @nickelpro expressed strong resistance to that, we decided that the most reasonable approach we can both agree on is to provide instead <name>:<name> and possibly <name>:<name>-private.

(Another point raised was that the provider name should be pkg-config, not pkgconf.)

At this time, that is what we plan to implement in CMake...

Probably we can also say that pkg-config provided packages will ignore components on the <requirement>, since there aren't really such a thing. (Again, the original idea for resolving libraries would have made it reasonable to require those components, since that would be equivalent to saying 'fail if you can't actually find a shared library', but under the revised proposal, I think it makes the most sense to always provide all possible components — all two, at this time — even if they may be identical.)