Use the new unstable meson cargo support. This simplifies the
meson.build script and allows to use crates such as clap that require
picking up features from Cargo.toml.
This also allows us to not embed thiserror in subprojects, and instead
to simply place some patches and a meson.build in a specific directory
and meson will automatically pick them up.
The new version of mon fixed the SeparatedBy combinator to not allow
trailing delimiters. This broke the Depend expr parser, because the
exprs are padded with whitespace. Using the new
SeparatedByWithTrailing combinator fixes this issue.
