In order to support generation of Common Package Specifications, the
mechanisms CMake uses to export package information need to be made more
abstract. The prior commits began this refactoring; this continues by
(actually) restructuring the classes used to generate the actual export files.
To minimize churn, this introduces virtual base classes and
diamond inheritance in order to separate logic which is format-agnostic
but depends on the export mode (build-tree versus install-tree) from
logic which is format-specific but mode-agnostic.
This could probably be refactored further to use helper classes instead,
and a future commit may do that, however an initial attempt to do that
was proving even more invasive, such that this approach was deemed more
manageable.
While we're at it, add 'const' in more places where possible.
In order to support generation of Common Package Specifications, the
mechanisms CMake uses to export package information need to be made more
abstract. As a first step toward this, refactor cmInstallExportGenerator
so that logic specific to config.cmake and Android .mk lives in separate
subclasses.
While we're at it, clean up the code style a bit and try to use moves a
bit more consistently.
This is step 1 of 2. The next step will refactor the individual file
generators along similar lines, which will also involve creating
additional classes for format-agnostic logic that is shared between
build-tree and install-tree variants.
