Suppress needlessly alarming messages

[dabrahams]

These packages are not marked REQUIRED and when this project is used as a dependency of another CMake project they don't need to be findable when this CMakeLists.txt is read. They may in fact be found later in the configuration process, so the messages when they actually are needed, so the messages

-- Could NOT find dispatch (missing: dispatch_DIR)
-- Could NOT find Foundation (missing: Foundation_DIR)
-- Could NOT find XCTest (missing: XCTest_DIR)

are needlessly alarming.

Replace this paragraph with a description of your changes and rationale. Provide links to an existing issue or external references/discussions, if appropriate.

Checklist

• [x] I've added at least one test that validates that my change is working, if appropriate
• [x] I've followed the code style of the rest of the project
• [x] I've read the Contribution Guidelines
• [x] I've updated the documentation if necessary

[compnerd]

@swift-ci please test

[etcwilde]

Yeah, I'd prefer to keep the logging. If they aren't a WARNING, and aren't an ERROR, they won't prevent you from continuing and are just informational, but if they aren't emitted and you were expecting to find them, having that in the log is definitely preferable to scratching your head wondering why things were getting built weirdly.

[natecook1000]

Maybe instead of marking these QUIET, we could add an additional message that contextualizes the output?

[dabrahams]

Maybe you could conditionally find_package based on the conditions that actually require them, and under those conditions make them REQUIRED? The current situation:

• For developers of this package, defers errors to link time with a much harder-to-understand message, after you've wasted a bunch of time proceeding past an unrecoverable condition
• For consumers of this package, produces confusing messages—in my case, for example XCTest actually is found, only later. So it's not just alarming, it's misleading to see this message.