By "flatten" I mean that component libraries aren't referenced by
genex anymore; instead, their include directories and libraries
are copied out of their interfaces and into the interface of
the metalib.
This prepares for exporting our targets, by making sure that the
component libraries don't have to be installed for CMake to be
able to export metalibs.
Instead, let's use a PKG::PKGNAME interface library, which simplifies
the linking and also allows us to use imported libraries from
find_package in the future.
They export no code, and are really only used to contain several
preprocessor macros relevant to their respective packages. On
Windows, they're a problem: MSVC doesn't generate a .lib when
compiling a .dll that exports nothing.
This fixes some cases where transitive libraries, as nominated
by a component library, were not being absorbed into a metalib,
due to the library actually not being defined yet. This makes the
logic a little more robust, first by explicitly requiring a metalib
definition to come after all component library definitions, and second
to assume a library "linked" to a component library is a non-component
if it isn't defined (yet).
