Handle pugixml as any other dependency

[traversaro]

I think as for all other dependencies of the robotology-superbuild software, we should try to make sure that all projects included in the robotology-superbuild superbuild use the same version of pugixml, to avoid maintanance problem of different kind in the future.
At the moment, the only dependencies that uses pugixml vendoring it is blocktest, and we should pass to it the BLOCKTEST_USE_EXTERNAL_PUGIXML https://github.com/robotology/blocktest/blob/a0df5f9293dd4c9e3bd15c21b6db49683086875f/CMakeLists.txt#L25 .

Unfortunately this is not trivial to do on Ubuntu 18.04, as the pugixml version available in the system has broken CMake config files. For this reason, probably it make sense to revisit this once we drop support for Ubuntu 18.04 (that will not happen anytime soon, I am afraid).