port to meson cargo

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
use a wrap file with a custom meson.build and some patches to make it
compile without running its build.rs script.

.docker/Dockerfile

@@ -2,8 +2,6 @@ FROM gentoo/stage3:latest
 
 COPY ./. /
 
-ENV EGIT_CLONE_TYPE=shallow
-
 RUN getuto
 
 RUN emerge-webrsync
@@ -16,14 +14,13 @@ RUN emerge \
     sys-process/parallel \
     net-libs/nodejs
 
-
 RUN git clone https://jturnerusa.dev/cgit/ebuilds/ /var/db/repos/spawns
 
 COPY etc/portage/repos.conf /etc/portage/
 
 RUN mkdir -p /var/cache/distfiles/git3-src && chown portage:portage /var/cache/distfiles/git3-src
 
-RUN emerge =dev-build/meson-9999
+RUN emerge =dev-build/meson-9999::spawns
 
 RUN useradd -m gentooligan