ViJay's answer worked, after I clicked on a context popup in the source code. The red mark was removed on the import line in the source and the project works. Calls org.codehaus.plexus.util.DirectoryScanner. It was confusing that the red mark on the import statement did not disappear right away. I don't remember what the context popup said but it fixed it. Apparently downloaded the library and installed it.

Who knew. It's all just magic.