OSGi: move plugin localization to subdirectory

OSGi: move plugin localization to subdirectory

OSGi can have its plugin localization at an arbitrary place; there is
no need to have it in a top-level plugin.properties file. In non-OSGi
environments having the files at the root level may mean that these
files clash with each other, or, as in the referenced bug, with some
third-party plug-in's plugin.properties, which may not even have
anything to do with localization.

Move our OSGi localization to a subfolder OSGI-INF/l10n. For OSGi
environments, that's just as good, and for non-OSGi environments it
avoid clashes with other root level items on the classpath or in a fat
JAR.

For fragments, use neither plugin.properties (which would clash with the
host plug-in's plugin.properties) nor fragment.properties (which might
clash with other fragments for the same fragment host bundle). Instead
use names "relative" to the host bundle.

Bug: 582394
Change-Id: Ifbcd046d912e2cfe86c0f7259c5ca8de599d9aa1
Signed-off-by: Thomas Wolf <twolf@apache.org>