Developer checkouts are a special setup intended for Horde developers.
A developer checkout will contain all mainstream and alpha applications, libraries etc directly from git.
A special helper tool git-tools is used for managing the large number of git repos.
The developer setup consists of two different hierarchies: The git checkout and the web-readable tree.
The git checkout is a directory containing all Horde libraries (capital letters) and applications (lower case letters) as seen on github.
The web readable tree is a re-arranged structure of symlinks to simulate what would be seen in a pear installation.
This allows combining original, bleeding edge or downstream patched components in desired combinations.
If you are rather looking for a simple setup, try PEAR or composer.