Install bash completion with correct name

Move the completion to completions/bash/apt and install all
bash completions from completions/bash.

Gbp-Dch: ignore

debian/apt.install.in

@@ -4,4 +4,4 @@ bin/methods/* usr/lib/apt/methods/
 scripts/dselect/* usr/lib/dpkg/methods/apt/
 usr/share/locale/*/*/apt.mo
 bin/libapt-private.so.* usr/lib/@DEB_HOST_MULTIARCH@/
-../debian/apt.bash-completion /usr/share/bash-completion/completions/
+../completions/bash/* /usr/share/bash-completion/completions/