export: Implement EXPORT subcommand (#9822)

Teach the export command to handle export sets defined by invocations
of install(TARGETS ... EXPORT foo).  This makes maintenance of targets
exported to both the build tree and install tree trivial.
Help: cmake-packages: Add missing slash.
Don't copy find_dependency in configure_package_config_file.

There is not really any need to. Downstreams can either rely on it
being provided by CMake, or copy and distribute it.

Change the documented include for the find_dependency macro.
Help: Document export(EXPORT) in the cmake-packages manual.
Merge topic 'export-EXPORT-subcommand'

98b9f52 Help: Document export(EXPORT) in the cmake-packages manual.
a1d2bda Don't copy find_dependency in configure_package_config_file.
f4f6529 Help: cmake-packages: Add missing slash.
cbe7e8f export: Implement EXPORT subcommand (#9822)