Revert "Avoid using temp_await on loadRootPackage"

[MaxDesiatov]

Reverts #7009 to fix macOS CI regression:

duplicate symbol '_async_MainTu' in:
    /Users/ec2-user/jenkins/workspace/swift-package-manager-with-xcode-self-hosted-PR-osx/branch-main/swiftpm/.build/x86_64-apple-macosx/debug/swift_package.build/Entrypoint.swift.o
    /Users/ec2-user/jenkins/workspace/swift-package-manager-with-xcode-self-hosted-PR-osx/branch-main/swiftpm/.build/x86_64-apple-macosx/debug/swift_test.build/Entrypoint.swift.o
duplicate symbol '_async_Main' in:
    /Users/ec2-user/jenkins/workspace/swift-package-manager-with-xcode-self-hosted-PR-osx/branch-main/swiftpm/.build/x86_64-apple-macosx/debug/swift_package.build/Entrypoint.swift.o
    /Users/ec2-user/jenkins/workspace/swift-package-manager-with-xcode-self-hosted-PR-osx/branch-main/swiftpm/.build/x86_64-apple-macosx/debug/swift_test.build/Entrypoint.swift.o
ld: 2 duplicate symbols for architecture x86_64

This bug was fixed in Swift 5.10, which is not available on CI yet.

[MaxDesiatov]

cc @AndrewHoos

[MaxDesiatov]

@swift-ci test

[MaxDesiatov]

Actually we may get away with reverting only #7196, since that will keep a single simple async main symbol in the final binary, which shouldn't trigger linker errors on its own. And I'd rather prefer changes in this PR to stay, as they're more impactful touching the Workspace API surface.

[MaxDesiatov]

Closing, as it won't be needed after #7196 is merged.