Temporary dependency-reduced-pom.xml in basedir causes problems within shared build trees

The runtime dependency-reduced-pom.xml (not the one in outputDirectory) causes problems within a build tree that is shared across multiple machines/builds. This file is cleaned up after it is no longer needed and if more than one build is running shade and the subsequent goals within the same subdirectory at around the same time, the dependency-reduced-pom.xml file can be wiped out by one process while another process still requires it.