Change the interface of the `export` command so that `PACKAGE_INFO` and
`SBOM` are proper, first-level sub-commands, rather than being implemented via
`export(EXPORT ... (PACKAGE_INFO|SBOM) ...)`. This brings consistency with the
`install` command and also simplifies argument parsing.
Update the documentation and test cases accordingly. Some of the
argument handling test cases are now rather gratuitous, and are removed,
which also improves consistency with the `install` tests.
Add version validation to cmExportPackageInfoGenerator. Specifically, if
a package uses the "simple" version schema (which is the default if no
schema is specified), validate that the specified version actually
conforms to the schema and issue an error if it does not. Also, issue a
warning if the schema is not recognized.
Matthew Woehlke