support multiple instances of the same logical module

it is desirable to support the case of several instances of the same logical module (i.e., modules with the same name) competing with each other within a Product's context.

fundamentally, there are two axes along which these instances can be distinguished:

1. content (QBS-995): the modules have the same name, but differ by other properties which limit their suitability for a particular build. some such properties are:
   • everything that affects ABI (target OS and architecture, toolchain vendor and version, exception model, etc.)
   • build options that enforce dependencies, e.g. debug vs. release runtime on Windows
   • build options that disable optional features
2. source (QBS-1604, QBS-1658): the modules differ only by where they come from:
   • a project may want to use a library which is already installed on the system, but still have an own copy of the library to have a fallback - think libpng in qt
   • the system's library may be provided by several modules, like a pkg-config based probe, and a "hand-crafted" one ("inline" in the new qt configure's terms).
   • external libraries will ideally come with own qbs files, one for each build.

approximate mechanism proposal

• qbs iterates over products, collecting dependencies
• each dependency must be satisfied by a module or product
• which of these it should be (preferentially) is decided by examining qbsModuleSources (QBS-1658)
• if a product is chosen, that may cause it to be built in the first place (QBS-581) and decide about how it is built (QBS-1197)
• if a native module is chosen, qbsSearchPaths is scanned for it
• if a generated module is chosen, a provider needs to generate it (QBS-1107)
  • which provider(s) are used is decided by examining qbsModuleProviders (QBS-1604), and conversely, providers may be told which modules to actually generate (QBS-1662)
  • providers may be configured by packages (QBS-1452)
  • logically, all providers are executed, and the first module in the list (ordered by qbsModuleProviders) that matches the requirements (QBS-995) is used, though for efficiency the providers may be fed the desired module properties as well, so unsuitable modules can be omitted from generation a priori, and the search can be cut short after the first match