Name already in use

Resolves: #108

The Meson and LDC issues break everything currently, and it will take a
while to get sorted out.

Previously we only allowed removing components, now the repository
vendor can also inject arbitrary additional data into <custom/> tags.

The only current limitation is that data needs to be reprocessed for a
particular component so changes to a <custom/> tag are applied.

Previously, the screenshot storage, metainfo artifact propagation and
custom entry filtering flags were ignored by accident.

Since Fedora Linux 36, there is now an ffmpeg implementation provided
in the main repositories, so let's use it.

We do not need it for anything anymore.

Libarchive may use more space than D fibers give by default, so we just
increase the fiber stack size and hope that this will not cause any
trouble later when we are reading a lot of archives in parallel.

Resolves: #101