export: Factor out CMake-specific export generation (1/2)

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.

Source/cmExportFileGenerator.h

@@ -207,6 +207,8 @@ protected:
 
   virtual cmExportSet* GetExportSet() const { return nullptr; }
 
+  std::string GetCxxModuleFile(std::string const& name) const;
+
   void SetRequiredCMakeVersion(unsigned int major, unsigned int minor,
                                unsigned int patch);