Automatically use source package's architectures in non-native mode

[lengau]

Currently non-native mode doesn't check the source package's control file to determine the platforms, instead
just defaulting to the standard platforms that are roughly equivalent to any.

In non-native mode we should refer to the debian/control file to figure out what platforms we need.

As a note, the special cases of any all or <arch> all can be taken to simply mean any in the platforms key, since we will build those architecture-independent packages in any build (unless a package subset is declared).