Cleanup MANIFEST.MF in JGit

- We shouldn't be re-exporting bundles, JSch in this case.
  Instead used a Import-Package clause.

- Some packages weren't exported.  In Eclipse we have a policy to
  export all the packages in a bundle

- Exporting version numbers on the export packages.

Bug: 291108
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>