Restructure binding module code gen

[ZacSweers]

This PR reimplements binding module generation to perform all merging at the IR level and eliminate intermediate merged binding modules.

At a high level, this does the following

• Updates @ContributesBinding and @ContributesMultiBinding to instead generate binding modules per-binding.
• Each binding modules is annotated with a new @InternalBindingMarker annotation that contains a minimal amount of metadata
  • originClass - the origin class that contributed the binding
  • isMultibinding - indicates if the binding is a multibinding
  • priority - corresponds to ContributesBinding.Priority.
  • qualifierKey - the computed "qualifier key"
• Each binding module is annotated with @ContributesTo and propagated via that infrastructure.
• During ModuleMergerIr, these binding modules are merged and metadata read from @InternalBindingMarker to properly allow for excludes, replaces, priority, etc all work correctly.
• Update tests for this new infrastructure, preserving as much of the existing binding module tests (unit and integration) while removing only now-irrelevant ones (i.e. ones that explicitly expect a BindingModule of the old style to always be generated).

This also functionally completes #751!

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• [x] Update ContributesBinding and ContributesMultibinding to generate contributed binding modules
• [x] Update ContributesBinding and ContributesMultibinding tests for new impl
• [x] Aggregate contributed bindings in ModuleMergerIr + new modeling using @InternalBindingMarker
• [x] Finish reimplementing prioritization and replacements in ModuleMergerIr. This is a little tricky because BindingModuleMerger speaks classes but ModuleMergerIr speaks modules.
• [x] Update tests