Deal with Xtext leaving the simrel

[cdietrich]

Xtext plans to leave eclipse simrel https://github.com/eclipse/xtext/issues/1961

we currently consume xbase.lib and xtend.lib at runtime from them.

• (re)package it on our update site
• get rid of the dependencies at runtime (needs to be checked if it is used in impl only or in api too). i also wonder how we can test this (xtend/xbase lib api not used at runtime). we also need to find out how to remove it from the manifest

[cdietrich]

Besides Pure and ToStringBuilder https://github.com/eclipse/lsp4j/issues/494 there are calls to collectionliterals

[cdietrich]

xbase/xtend.lib no longer is a runtime dependency