`package` autocompletion should return only 1 package

[fbricon]

After creating a new java file in a source directory, autocompletion after the package keyword should return the current package, inferred from the file location. Instead, a complete, useless list of packages is returned.

[fbricon]

Created upstream issue https://bugs.eclipse.org/518140

[fbricon]

completion improvement with eclipse-jdtls/eclipse.jdt.ls#260