Build: xml: upgrade-*.xsl: ship run-time used (& only such) stylesheets

Previously, "make dist" would omit those(!), and over-approximation
of such files would get, undesirably, selected (since b2987ac2a
incl. upgrade-detail.xsl) in the respective source variable.
Also newly added upgrade-2.10-roundtrip.xsl is not intended
for distribution.