Debian packages should not install into /srv. The specification of /srv states that its structure is at the discretion of the local administrator and no package should rely on any particular structure. Debian packages that install files directly into /srv can't adjust for local policy about its structure and in essence force a particular structure.
If a package wishes to put its data in /srv, it must do this in a way that allows the local administrator to specify and preserve their chosen directory structure (such as through post-install configuration, setup scripts, debconf prompting, etc.).