Always generate unique doc-base file names (Closes: #980903)

Always use package name and document ID.

With these changes, the zsh package mentioned in #980903 builds these two doc-base files:

$ debc | fgrep zsh-faq
-rw-r--r-- root/root       504 2020-06-30 17:42 ./usr/share/doc-base/zsh-common.zsh-faq
-rw-r--r-- root/root       519 2020-06-30 17:42 ./usr/share/doc-base/zsh-doc.zsh-faq

Upgrading to these packages from the current zsh-built binary packages in Debian Unstable/Testing (5.8-5) causes the doc-base trigger to emit this output:

Processing 2 removed doc-base files, 3 added doc-base files...

The files generated by doc-base are:

$ ls -l /var/lib/doc-base/documents/zsh*
-rw-r--r-- 1 root root 519 Jan 24 06:54 /var/lib/doc-base/documents/zsh
-rw-r--r-- 1 root root 613 Jan 24 06:54 /var/lib/doc-base/documents/zsh-faq

And the two zsh-*.zsh-faq files are indeed merged into /var/lib/doc-base/documents/zsh-faq.