Spinning wheel when importing a XAR when the package.xml file is invalid

When you select your XAR in the import UI, you get the spinning wheel that never stops. No error is printed in the logs nor on the screen.

In my case I had to debug XarScriptService#getXarPackage() to find that the error was:

org.xml.sax.SAXParseException; lineNumber: 34; columnNumber: 3; The element type "files" must be terminated by the matching end-tag "</files>".