Detect apt pinning holding a package version back

[lathiat]

The pro client pins UbuntuESM to Pin-Priority 510, overriding not only the Ubuntu archive (as intended) but all PPAs including the OpenStack Cloud Archive. As a result, any package with an ESM fix will install the ESM version, even if a much newer major version of the software is in a PPA.

Additionally, even if the new version was installed before Pro client enabled the pinning, it will prevent upgrading to newer versions in the PPA, and apt won't tell you about this.

We should try to detect this situation from the apt-cache policy output, to help support engineers realise they MAY have the wrong version of a bit of software (e.g. wrong major version, or missing an SRU in the PPA)

More details here: canonical/ubuntu-pro-client#3330