Merge topic 'file-set-simplifications'

9916d4dd44 cmTarget: factor out fileset type handling
79d6b928a3 RunCMake/target_sources: test `HEADERS` file sets via generic props
d74f9599f6 cmTarget: require filesets to be of the right type
5da4fe30a9 cmTarget: factor out fileset property manipulation
c89580487b cmTarget: pass candidate strings by const-ref
64ea1a272c messages: remove screamake from comments and errors

Acked-by: Kitware Robot <kwrobot@kitware.com>
Tested-by: buildbot <buildbot@kitware.com>
Merge-request: !7145