cmGeneratorTarget: Add methods to access source file groups.

These methods and others will be able to get a config parameter
later to implement the INTERFACE_SOURCES feature.
cmTarget: Make GetSourceFiles populate an out-vector parameter.

In a future patch, this will also be populated with extra
sources from the linked dependencies.
Merge topic 'minor-cleanups' into next

531e40b cmTarget: Make GetSourceFiles populate an out-vector parameter.
38de54c cmGeneratorTarget: Add methods to access source file groups.
Re-word policy doc.
Remove note about usr-move.
Merge topic 'extend-docs' into next

15d7f7f Remove note about usr-move.
381282f Re-word policy doc.