Refactor Metatypes dependency propagation

Every metatypes.json files is now added as an INTERFACE source file to a
target. This enables us later to correctly collect all the
metatypes.json files from dependent targets. This information is also
correctly exported via export()/install().

To avoid the metatypes.json appearing in every target's source list,
the file path is wrapped in a generator expression which will only be
evaluated when the consuming target has the property
QT_CONSUMES_METATYPES set to true. At the moment this is limited to
targets which need to interact with qmltyperegistrar.

Change-Id: I0ffebcd069a923383f7ed11cde2c94ecf2fb13f3
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>