#1330: Fix crash when assembling package tags

In a recent refactoring we removed all notion of string based tags
but put them in TagDescriptors. Apparently I missed a bit of handling
of the package tags and this caused fatal errors later due to typehints.
Merge pull request #1334 from mvriel/feature/1330

#1330: Fix crash when assembling package tags
Update Extension.php

Fixes #1326
Update version to 2.6.1 and update changelog
Merge branch 'master' of github.com:phpDocumentor/phpDocumentor2 into develop