To manage the generation step, introduce cmGeneratorFileSet and
cmGeneratorFileSets classes.
These reorganizations of the code are done in preparation of the
implementation of SOURCES file set type as well as user's file set types.
Issues: #27550, #27383
The cmLinkImplItem class used to contain some additional members
to support the CMP0027 policy. Since the CMP0027 policy and those
associated members were removed, the cmLinkImplItem class has been
a plain wrapper around cmLinkItem with no additional members.
It is no longer needed and cmLinkItem can be used directly wherever
cmLinkImplItem was used previously.
Since commit 1e49880472 (cmGeneratorTarget: Avoid boolean trap in usage
requirement lookup, 2021-12-08, v3.23.0-rc1~245^2) we have clarified
the distinction between compile-only and link-only usage requirements.
Rename the `LinkInterfaceFor` enum to `UseTo` to clarify that its role
is to specify the purpose of usage requirements.