Centralize definition of platform requirements

Is your feature request related to a problem? Please describe.

Project versions currently have three ways of requiring things:

The generator's automatic analysis yields required feature triples nature:name[:version]
The recipe (or a template) can contain extra-requires definitions also specifying feature triples
The recipe (or a template) can contain platform-requires definitions specifying lists of systems packages in a ancestry-based tree of platforms:
```
platform-requires:
 ubuntu:
   packages:
     - package1
   xenial:
     packages:
       - package2
       - package3
```

Requirements of kinds 1. and 2. are resolved between project versions in the distribution under consideration to produce concrete (ordering) dependencies. Requirements of kind 3. are used to compute a list of required system packages for a given target platform.

See this presentation for more details.

There are two problems with this approach:

Requirements of kinds 1. and 2. cannot be resolved using system packages. Imagine the automatic analysis somehow yields the required feature pkg-config:libsomething:1.2.3. Even if there is a platform-requires entry for say the libsomething-dev package, the required feature will still be considered unsatisfied.
A requirement can be satisfied by system packages in different ways. For example, depending on a fictional "Library for something" in a C++ program could correspond to the following requirements on the system package level:
```
platform-requires:
 ubuntu:
   xenial:
     packages:
       - libsomething2-extra
       - libsomething2-dev
   bionic:
     packages:
       - libsomething3-dev
 redhat:
   packages:
     - libsomething-devel
```
This is not a problem when specified in a single recipe, but imagine multiple projects depending on this library. Each library would have to include this fragment. Obviously not good.

Describe the solution you'd like

Introduce a new platform-provides variable that can be used to describe the provided features of system packages. The syntax is as follows:

platform-provides:
- name: Protocol Buffers C++ Library
  variables:
    extra-provides:
    - nature: cmake
      target: ProtocolBuffers
    platform-requires:
      ubuntu:
        xenial:
          packages:
          - libprotobuf1-dev
        bionic:
          packages:
          - libprotobuf2-dev
⋮

A list of these descriptions would be collected and maintained in a template (solving problem 2.). A recipe currently containing

platform-requires:
 ubuntu:
   xenial:
     packages:
     - libprotobuf1-dev
   bionic:
     packages:
     - libprotobuf2-dev

could then be simplified to

extra-requires:
- nature: cmake
  target: ProtocolBuffers

or even nothing if the feature requirement can be identified by the automatic analysis (solving problem 1.).

RDTK / generator

Centralize definition of platform requirements #16