Optimise XPATH expression resolving

[metacontext]

Currently there are too many places where XPath expressions are resolved. This needs to be cleaned up. Therefore:

• check if expressionBasedUriResolver and expressionBasedAttributeResolver
  • don't do the same
  • have $repeatIndex and $resourceIndex replaced in all expressions parsed

Then:

• switch to new function => attributeContentExpressionResolver
• switch to new function => tagContentExpressionResolver