When registering a new tag library, you would usually declare the facelets taglib file in the web.xml configuration file.
As this parameter can only be declared once, and is already declared in the nuxeo base ui module, you cannot declare it using the Nuxeo deployment feature.
A workaround is to put you custom taglib file mylibary.taglib.xml in the META-INF folder of your custom jar: it will be registered automatically (even if it triggers an error log at startup).
As a reminder, the tag library documentation file, mylibrary.tld, is usually placed in the same folder than the taglib file, but it is only used for documentation: it plays no role in the tags registration in the application.