Try:

apt-cache depends <package-name>  |  grep gtksourceview

With Mousepad, I get this:

$ apt-cache depends mousepad | grep gtksourceview

  Depends: libgtksourceview2.0-0

The others (Geany and Medit) don't depend on version 2.0 of GtkSourceView.

As a side note, to check which packages provide a directory/file, you can use dpkg -S <package-name> if the file is in your system or apt-file for the general case:

dpkg -S /usr/share/gtksourceview-2.0       # local file/directory

apt-file find /usr/share/gtksourceview-2.0 # works even if the file is not installed

Since apt-file is not installed by default you need to sudo apt-get install apt-file. With the above command, I get (among other results) the package libgtksourceview2.0-common. Now I can use apt-cache rdepends to seek for packages which directly/indirectly provide the directory:

$ apt-cache rdepends libgtksourceview2.0-common

libgtksourceview2.0-common

Reverse Depends:

  libgtksourceview2.0-0

  libgtksourceview2.0-0



$ apt-cache rdepends libgtksourceview2.0-0 | grep 'geany|medit|mousepad'

  mousepad

This also seems to work (and possibly more general):

apt-cache rdepends libgtksourceview2.0-common 

  --recurse 

  --no-recommends 

  --no-suggests | grep 'geany|medit|mousepad'