Problem solved! caster at b.g.o. suggested running

Must be some orphan file.
Try grep ant-junit /usr/share/*/package.env

which found CLASSPATH and DEPEND lines in .../ant-tasks/package.env.
After removing the ant-junit references, emerge gave a different error
message. I then disabled all the /usr/share/ant* directories by
renaming them to /usr/share/ant*.xxx

After running the following commands (with results shown):

1) emerge ant-junit failed saying "!!! ERROR: Package ant-core was not
found!"

2) emerge ant-core succeeded

3) emerge eclipse-sdk successfully built multiple packages until
dev-java/ant-apache-bcel-1.7.1 failed saying "!!! ERROR: Package
ant-nodeps was not found!"

4) emerge ant-nodeps succeeded

emerge eclipse-sdk has succeeded !!!

Conclusions:

1 - jar file references in /usr/share/ant-tasks/package.env caused the
error messages I didn't understand

2 - ant-nodeps is needed for ant-apache-bcel, but is incorrect in the
ebuild

3 - install problem has been solved