Match the update's summary and description with its title

[randomnoise]

When multiple updatable packages are grouped together as an update, the group uses its source in APT description as its title in GUI.

The title can correspond to a package itself but not necessarily be updatable at that moment. In this case, the summary and description of the first package in the changelist are used whose name doesn't begin with lib and end with dev, dbg, common, core, data or doc.

This PR uses the source package's summary and description to match with the group's title if the source package exists. If a package doesn't exist with the title's name, the code shouldn't cause an error. (e.g., updatable package libfprint-2-2, its source libfprint: no package by that name.)

This implementation doesn't feel ideal; I implemented it for minimal change in terms of lines, and I didn't want to introduce a circular dependency between AptUpdater (old APTCheck) and Update classes. I just wanted to explore the root cause of the issue, that's why it is kind of hacky.

• Closes Wrong description shown #806

vim specific explanation

$ apt-cache show vim-common | grep Source
Source: vim

but updatable changes doesn't include vim, so GUI uses first acceptable change for the summary and description: xxd.

updatable packages:

[
 <Package: name:'vim-gui-common' architecture='amd64' id:136982>,
 <Package: name:'xxd' architecture='amd64' id:10585>,
 <Package: name:'vim-gtk3' architecture='amd64' id:11795>,
 <Package: name:'vim-common' architecture='amd64' id:10066>,
 <Package: name:'vim-runtime' architecture='amd64' id:11791>
]

vim before & after screenshots

Before:

After: