Support loading development packages in makeExperimentRegistry

[renozao]

I usually write R code in packages, straight from the beginning of a project.

It would be nice to be able to provide path to required packages that are to be loaded using devtools::load_all.
I can think of the following options to pass package base directories to makeExperimentRegistry:

• add an extra argument e.g., src.pkgs
• allow passing the path in existing argument packages, with names starting with a ./ or / being interpreted as source packages and loaded with devtools::load_all. This would make it very easy to switch from a source package to an installed package, once development/debugging is done or when running on production mode: just remove the leading path piece of the name.
• allow the path to be passed in src.dirs, where paths that contain a DESCRIPTION file will be interpreted as source pacakges.
• allow all of the above :)