[PD] search for installed externals (was: Re: detect dependencies from patch)

[Fede Camara Halac]

This would be a nice feature to have within the deken plugin: to search among already installed externals

> however, it also means that deken doesn't know anything about which libraries are already installed, and which versions thereof.

Currently, you can browse from pd's browser, but it involves going back and forth between deken and the pd browser.