The full path of a file in a file set can be broken into groups like
`<base>/[<sub>/]<name>`, where `<base>` is the base directory and
`<name>` is the file name without any directory components. Existing
users evaluate file set files as a mapping from `<rel>` to the full
path. However, CPS export needs files grouped by base directory, and
cares about the composition of `<rel>` and `<name>`. To facilitate
obtaining this information in a more useful form, refactor how we
extract file set files to allow the user to pass a lambda 'handler' for
each file, which has the base directory, full path, and path relative to
the base directory available. (We already obtained `<rel>` from the
latter, so while this technically means the existing use needs to do
additional work, in practice it just means the call to get `<rel>` moves
to the lambda.)
Because there are many existing users that would need to duplicate the
lambda, the old methods are retained as-is, although this does result in
minor code duplication between `GetFiles` (which calls the overload of
`EvaluateFileEntry` which fills the old file map) and `EvaluateFiles`
(which is essentially `GetFiles`, but accepts a user-provided 'handler'
lambda rather than directly producing a file map).
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
Marc Chevrier
Matthew Woehlke