Versioned sonames are normally used to indicate backwards compatibility in regards to the ABI. For mupdf, upstream does not use versioned sonames, meaning upstream doesn't guarantee ABI stability. The versioned soname is added downstream by us. However, since upstream doesn't guarantee ABI stability we need to rebuild all packages depending on libmupdf with each upgrade anyhow. Therefore, the versioned soname is of no use and—to the contrary—causes quite a bit of confusion as it makes people incorrectly believe that a rebuild is not necessary even though it is.
Other distributions (Debian, Arch, Gentoo, …) for this reason also do not ship libmupdf with a versioned soname and just leave it as is.
This change therefore also removes the -libs subpackage and provides libmupdf.so in the -dev subpackage.