XslTransformation Locks Input Files

[benvillalobos]

Caused by #6863

Issue Description

Following #6863, the created XmlReader is no longer responsible for its underlying stream. This can cause the build process to hold on to the processed file, preventing its removal. This can especially be a problem when the transformation is in fact aimed at the input file itself, where we want to create the transformed file, then move it to the original.

Will be fixed by #6946