the 'qbs' module should be a namespace instead

"qbs" is currently an actual module. but because modules can be imported into scope only via Depends items, it is not accessible inside Project items. this is inappropriate for something that represents global settings, and leads to unnatural workarounds like the Project::qbsSearchPath+Product::qbsSearchPath combination.

so instead, "qbs" should be only a namespace for actually global properties.

to enable this, several properties which apply to build targets must move to the cpp module first; see QBS-1611.