[Build] NFC: Compute per-module macro requirements on-demand

[xedin]

Instead of having to go over all of the products and establish module -> product relationships during build plan construction let swift module build descriptions compute required macros based on their underlying modules as necessary.

Motivation:

Build plan is required to pre-compute macro requirements but that's expensive and shouldn't be necessary.

Modifications:

• Instead of passing macro products to SwiftModuleBuildDescription all we need to pass are build parameters to use for macros and the macros themselves could be found based on the underlying module.

Result:

Build plan construction is slightly simpler and less expensive to compute, product and module descriptions could now be computed independently.

[xedin]

@swift-ci please test