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.

bootstrap

@@ -408,6 +408,7 @@ CMAKE_CXX_SOURCES="\
   cmIncludeGuardCommand \
   cmIncludeDirectoryCommand \
   cmIncludeRegularExpressionCommand \
+  cmInstallCMakeConfigExportGenerator \
   cmInstallCommand \
   cmInstallCommandArguments \
   cmInstallCxxModuleBmiGenerator \