Don't install package itself when installing package's dependencies

[ceball]

When installing dependencies for package X, it's possible one of the dependencies of package X has package X itself as a dependency, in which case pytdev installs package X before going on to do a pip editable install of package X later.

E.g. panel lists holoviews as a recommended dependency, and holoviews lists panel as a dependency. pyctdev develop_install -o recommended for panel installs panel, but should not (it's subsequently installed in editable mode by pip - could be confusing).