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Actually, when it's deployed it is the manifest that determines who can
see what.
a plugin declares it's internal classpath and what packages it exports.
Another plugin just needs to say it depends on the first plugin.
If you have multiple jars in a plugin, the correct order of operations
is to update the manifest so the Bundle-Classpath is correct within that
plugin, and then go to the project popup men>PDE Tools and update
classpath. The .classpath is used for compiling and running within eclipse.