[BuildPlan] Switch build plan construction to use graph traversal

[xedin]

Motivation:

This way we can compute destinations as we go instead of relying on buildTriple computed during graph construction.

Modifications:

• Adds a new static method on BuildPlan to traverse modules graph and compute destinations for each product and module.
• Switches BuildPlan.init from using allProducts and allModules flat lists over the new incremental destination computation.

Result:

Removes one of the last places were buildTriple is required.

[xedin]

Next up we need to switch productMap and targetMap in BuildPlan to use IdentifiableSet and remove use of ResolvedModule.ID cannot do that without some sourcekit-lsp changes first.

[xedin]

@swift-ci please test

[MaxDesiatov]

Thank you! 🙏