Lektor projects may not want all packages in lego

The way most of the Lektor website projects are built, with packages symlinked to lego/packages is not ideal. For example, the i18n plugin is present on projects which do not need it, and when the plugin started to misbehave, it affected all projects and not only the ones that need i18n.

If projects instead symlinked packages selectively from those in lego/packages we would avoid this, and gain the ability for projects to use their own plugins without affecting the other sites (eg. index-pages for the blog project).

Furthermore it would probably be wise to revisit just why exactly some packages are in there, such as environs.