[Incremental Builds]optimize performance for ModuleDependencyGraph by reducing the number of external dependencies from N*M to N+M

[WeijunDeng]

Swift driver version

swift-driver version: 1.115 Apple Swift version 6.0.2 (swiftlang-6.0.2.1.2 clang-1600.0.26.4)
Target: arm64-apple-macosx14.0

Backgound

A swift module has 2500 swift files and 10000 external dependencies. These external dependencies include over 100 swift modules, along with a majority of header files. Thus, the graph.nodeFinder.usesByDef has 2500 * 10000 nodes of external dependencies，slowing down the Incremental build speed.

Improvement

	swift-driver total duration	planBuild	run(jobs:)	.priors file size	nodes number of external dependencies
Before optimization	33s	24.61s	6.11s	162M	N * M
After optimization	12s	8.50s	2.10s	53M	N + M
Optimization improvement	63.6%	65.5%	65.6%	67.3%	-

[WeijunDeng]

@swift-ci Please test

[WeijunDeng]

@swift-ci Please benchmark

[WeijunDeng]

@artemcm @cachemeifyoucan please review