Support explicit module paths

[rauhul]

Add support in the explicit modules mode for users passing the exact paths to their module dependencies.

Given the following build graph, where SDK represents an arbitrarily large subgraph:

  flowchart LR;
      A-->B;
      B-->D;
      A-->C;
      E-->B;
      D-->SDK;

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On the compilation of module "A" it should possible to pass the exact paths to .swiftmodule/.pcm files for modules "B", "C", and "D".

The driver will:

• Perform a dep scan.
• Not schedule module builds for B, C, D.
  • Verify the user passed modules are valid to be used and error if not.
• Schedule module builds for any other discovered modules.

[eschwieb]

Microsoft would love to have this feature as well. We'd prefer it in Xcode 26.0 if at all possible.