Resolve "Documents in default packages cannot be opened"

Since JavaCore.create(IFolder) can return either a package fragment or a package fragment root depending on whether the folder corresponds to a default package, the method signature needs to be changed. Callers must also check what they get and react accordingly. IPackageFragment.getPackageFragment needs an emtpy string to correctly identify default packages.

Closes #23 (closed)

enabled an automatic merge when the pipeline for 03fb71b4 succeeds

mentioned in commit abb6095c

merged