cmExportCommand: Port to cmSubcommandTable

Previously, the `cmExportCommand` parser did not use `cmSubcommandTable`
to distinguish between its `TARGETS`, `EXPORT`, `SETUP`, and `PACKAGE`
modes.  This created a maintenance burden when updating or adding
functionality.

Refactor the parser to use a subcommand for each keyword.  This new
structure mirrors the design of `cmInstallCommand` and simplifies future
development.