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/cmExportInstallFileGenerator.h

@@ -51,6 +51,9 @@ public:
     return this->ConfigImportFiles;
   }
 
+  /** Get the temporary location of the config-agnostic C++ module file.  */
+  std::string GetCxxModuleFile() const;
+
   /** Get the per-config C++ module file generated for each configuration.
       This maps from the configuration name to the file temporary location
       for installation.  */
@@ -118,6 +121,8 @@ protected:
   std::string GetFileSetFiles(cmGeneratorTarget* gte, cmFileSet* fileSet,
                               cmTargetExport* te) override;
 
+  using cmExportFileGenerator::GetCxxModuleFile;
+
   std::string GetCxxModulesDirectory() const override;
   void GenerateCxxModuleConfigInformation(std::string const&,
                                           std::ostream&) const override;