To really make Swift packages useful in scripts (which I'd also have many uses for!), we will need new features to the compiler still. The most recent discussions I could find were:
But let me try to answer this question, should it help you forward somehow!
Looking at what swift build -c release --verbose does under swift-argument-parser package, I can tell that SwiftPM passes the *.o files via indirection to swiftc when compiling the example apps. (Search for Objects.LinkFileList in the output.)
Assuming the command doesn't grow too large, you can also pass the object files directly; it looks like you'll need both ArgumentParser.build/*.o and ArgumentParserToolInfo.build/*.o.
Having first built swift-argument-parser, I was able to get your example to build and run with this command:
$ xcrun -sdk macosx swiftc \
-o example \
example.swift \
~/Desktop/SPMDownload/swift-argument-parser/.build/release/ArgumentParser.build/*.o \
~/Desktop/SPMDownload/swift-argument-parser/.build/release/ArgumentParserToolInfo.build/*.o \
-I ~/Desktop/SPMDownload/swift-argument-parser/.build/release/
$ ./example
Hello, World!
(I don't know what to make out of it, but would love to hear back if you do!)