make installed files actual artifacts in the build graph

currently, qbs essentially forgets about the files it has installed as soon as it's done.

apart from making various optimizations harder, this is also semantically unclean. in particular, it stands in the way of generalizing installs to "exports" which would make only selected files available to other Products in the Project (see also QBS-942). additionally, it would allow tagging the exported artifacts, which would make it possible for exported Rules to refer to them abstractly.

that means that the install commands (however specified) would be effectively taggers for an installer Rule.
the Rule itself can be executed multiple times for the same input files, in different contexts: an actual installation to an install root, a preparation of a device deployment (just creating a file mapping), an internal installation (to make a binary build tool runnable, compare QBS-583 in the context of QBS-590).