... to clean out all sorts of inconsitencies that can have accrued there over
time apparently due to how the mechanism of copying share/prereg/bundled worked,
and to work around stale $BUNDLED_EXTENSIONS_PREREG references in a better way
than the previous 81fd6b08 "fdo#53006 Force
reinstall of all bundled extensions on upgrade."

See the comment on refreshBundledExtensionsDir for how, at least in theory, that
functionality could be removed again in the future.

This effectively reverts 2d2b19de "Force
ExtensionManager resync when the implementation changes" and
81fd6b08 "fdo#53006 Force reinstall of all
bundled extensions on upgrade" (the latter at least on master; it had never been
cherry-picked to libreoffice-3-6), which it obsoletes.

Change-Id: I8f80c07a06ec9d53b03813338eeff7d7757c9d4d