bitbake-dev: Renamed to match the UI name

The UI starter now dynamically searches for the UI module based on the
parameter given to -u. bitbake advertises the dependency explorer as "depexp"
therefore this module needs renaming.

diff --git a/bitbake-dev/lib/bb/ui/depexplorer.py b/bitbake-dev/lib/bb/ui/depexp.py
similarity index 100%
rename from bitbake-dev/lib/bb/ui/depexplorer.py
rename to bitbake-dev/lib/bb/ui/depexp.py