'Prog/X/X11/' には、下位層のページがありません。
$ dpkg -l | grep dev | grep X11 ii libice-dev:amd64 2:1.0.9-2 amd64 X11 Inter-Client Exchange library (development headers) ii libsm-dev:amd64 2:1.2.2-1+b3 amd64 X11 Session Management library (development headers) ii libx11-dev:amd64 2:1.6.4-3 amd64 X11 client-side library (development headers) ii libx11-doc 2:1.6.4-3 all X11 client-side library (development documentation) ii libxau-dev:amd64 1:1.0.8-1 amd64 X11 authorisation library (development headers) ii libxaw7-dev:amd64 2:1.0.13-1+b2 amd64 X11 Athena Widget library (development headers) ii libxdmcp-dev:amd64 1:1.1.2-3 amd64 X11 authorisation library (development headers) ii libxext-dev:amd64 2:1.3.3-1+b2 amd64 X11 miscellaneous extensions library (development headers) ii libxmu-dev:amd64 2:1.1.2-2 amd64 X11 miscellaneous utility library (development headers) ii libxpm-dev:amd64 1:3.5.12-1 amd64 X11 pixmap library (development headers) ii libxt-dev:amd64 1:1.1.5-1 amd64 X11 toolkit intrinsics library (development headers) ii x11proto-core-dev 7.0.31-1 all X11 core wire protocol and auxiliary headers ii x11proto-input-dev 2.3.2-1 all X11 Input extension wire protocol ii x11proto-kb-dev 1.0.7-1 all X11 XKB extension wire protocol ii x11proto-render-dev 2:0.11.1-2 all X11 Render extension wire protocol ii x11proto-xext-dev 7.3.0-1 all X11 various extension wire protocol