What I usually do is, in Eclipse preferences, I define an Installed JRE called "JDK X.Y.Z" and include tools.jar as part of it libraries. Then I change the project preferences to use that JRE "JDK X.Y.Z".
Xerces used to be released as a single jar (xerces.jar), but was split into two jars, one containing the API (xml-apis.jar) and one containing the implementations of those APIs (xercesImpl.jar). Many older Maven POMs still declare a dependency on xerces.jar. At some point in the past, Xerces was also released as xmlParserAPIs.jar, which some older POMs also depend on.