textproc/libxml2*: update to 2.15.1, split -core

libxml2 now requires xsltproc from libxslt to build,
yet libxslt depends on libxml2,
so we need to break a dependency cycle.
documentation needed to be split out to a separate port.

libxml2 has therefore been split into two ports,

• libxml2 builds the documentation (unless disabled),
• library and executables in libxml2-core.

Pull in one contributor patch suggested by the former libxml2 maintainer
to address one downstream security issue in recent libxslt versions.

This should address arrowd@'s hint that installing libxml2 should
provide a full-featured package.

While here, deprecate libxml2-python according to upstream plans, we
have very few users in the ports tree, and expire end of 2026Q1.

PR: 291316