libapache2-mod-shib2: Installation fails because of unmet dependencies

[Jon Green]

Dear Maintainers,

I'm seeing the following error when trying to install the libapache2-mod-shib2 package. This causes the package to fail to install.

```
$ sudo apt install libapache2-mod-shib2
Reading package lists... Done
Building dependency tree
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 libapache2-mod-shib2 : Depends: libshibsp-plugins (= 2.6.1+dfsg1-2) but it is not going to be installed
                        Depends: shibboleth-sp2-utils (>= 2.6) but it is not going to be installed
                        Depends: libshibsp7 but it is not going to be installed
                        Depends: libxmltooling7 (>= 1.6.0-5) but it is not going to be installed
E: Unable to correct problems, you have held broken packages.
```

There are no "held broken packages" on this system.

System information:
```
$ lsb_release -rd
Description: Ubuntu 18.04.5 LTS
Release: 18.04
```

Package information:
```
$ apt-cache policy libapache2-mod-shib2
libapache2-mod-shib2:
  Installed: (none)
  Candidate: 2.6.1+dfsg1-2
  Version table:
     2.6.1+dfsg1-2 500
        500 http://us-west-2.ec2.archive.ubuntu.com/ubuntu bionic/universe amd64 Packages
     2.5.3+dfsg-2.1build1 -1
        100 /var/lib/dpkg/status
```

This error seems identical to an error reported for Debian Sid in 2018 (see link below).
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=909558

Best,
Jon Green