Warn about duplicate package-implementations
Warn about duplicate package-implementations

If two <package-implementation> elements generate implementations with
the same ID then log a warning. The Python version of 0install used to
do this, but the OCaml version was missing the check.

Reported in https://github.com/0install/repo.roscidus.com/pull/112
by pmiess.

0install/0install,0install/0install,0install/0install