[6.0] Cherry-pick recent NFC changes to reduce merge conflicts

[MaxDesiatov]

Includes these PRs cherry-picked off main

• https://github.com/swiftlang/swift-package-manager/pull/7605
• https://github.com/swiftlang/swift-package-manager/pull/7660
• https://github.com/swiftlang/swift-package-manager/pull/7667
• https://github.com/swiftlang/swift-package-manager/pull/7682
• https://github.com/swiftlang/swift-package-manager/pull/7687
• https://github.com/swiftlang/swift-package-manager/pull/7690
• https://github.com/swiftlang/swift-package-manager/pull/7684
• https://github.com/swiftlang/swift-package-manager/pull/7679

Explanation: Cherry-pick of recent NFC changes, which makes it easier to cherry-pick actual bug fixes onto 6.0 due to the reduced number of merge conflicts. Scope: broad, includes both modules graph and llbuild-related changes. Risk: low, the test suite is passing, no functional changes are included, and cherry-picked changes were incubated on main for some time. Testing: Existing automated test suite. Issue: N/A Reviewers: @xedin @MaxDesiatov @rauhul

[MaxDesiatov]

@swift-ci test

[MaxDesiatov]

CI seems to be blocked on swift-format errors apparently unrelated to this change.

[MaxDesiatov]

@swift-ci test