`swift test --explicit-target-dependency-import-check error` runs builds and executes tests even after discovering import error

[ahoppen]

For example, look at the Build / Test phase of https://github.com/swiftlang/swift-syntax/actions/runs/19796166265/job/56716763515?pr=3196.

--explicit-target-dependency-import-check error discovers the following error and prints it right at the start of the log:

error: Target SwiftWarningControl imports another target (SwiftDiagnostics) in the package without declaring it a dependency.

But then it continues to build the project and even execute the tests, which flushes the error all the way to the top of the log, making it hard to spot. I would propose that --explicit-target-dependency-import-check error should stop the build / test execution after discovering an issue.