Filterx expr location fixes

[bazsi]

This improves filterx expression representation in trace level logs by improving the location tracking for filterx source text.

This will also cause any source related displays (e.g. error output or the filterx expression display) to use the postprocessed output, e.g. the text after substitution, instead of the original text read from the source file.

This makes it easier to find issues that are related to parts substituted from parameters/environment variables.

This sits on top of the app-transform() branch