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' into next

402db78 Help: Document export(EXPORT) in the cmake-packages manual.
24ed23e Don't copy find_dependency in configure_package_config_file.
2fa48ab Help: cmake-packages: Add missing slash.
83fdce3 export: Implement EXPORT subcommand (#9822)