I've hit this issue before, but it took much effort to rediscover the answer, so I'm writing it down this time...
When you create a plug-in that simply packages and re-exports pre-existing JAR files, make sure that the following things are true:
- the JARs are all listed in the project build path ("Libraries" tab)
- the JARs have all been exported in the project build path ("Order and Export" tab)
- the JARs are all listed in the 'Classpath' list in the manifest ("Runtime" tab)
- the JAR packages that you wish to export (normally all of them) are listed in the export listof the manifest ("Runtime" tab)
- the JARs are all selected in the binary build list of the manifest ("Build" tab)
I had done all of these steps but #2. A dependent plug-in complained that package names could not be resolved -- even though all other signs indicated that the packages were accessible.