treewide: fix sourceRoot for fetchgit-based src

According to Nixpkgs manual[1] and NixOS 23.11 Release Note[2], the
`sourceRoot` attribute passed to `stdenv.mkDerivation` should be
specified as `"${src.name}"` or `"${src.name}/subdir"` when `src` is
produced using `fetchgit`-based fetchers.

`sourceRoot = "source"` or `sourceRoot = "source/subdir"` is based on
the assumption that the `name` attribute of these pre-unpacked fetchers
are always `"source"`, which is not the case. Expecting constant `name`
also makes the source FODs prone to irrelevent hashes during version
bumps.

[1]: https://nixos.org/manual/nixpkgs/unstable/#var-stdenv-sourceRoot
[2]: https://nixos.org/manual/nixos/stable/release-notes#sec-release-23.11

pkgs/development/python-modules/bootstrap/flit-core/default.nix

@@ -4,11 +4,11 @@
 , flit-core
 }:
 
-stdenv.mkDerivation {
+stdenv.mkDerivation rec {
   pname = "${python.libPrefix}-bootstrap-${flit-core.pname}";
   inherit (flit-core) version src patches meta;
 
-  sourceRoot = "source/flit_core";
+  sourceRoot = "${src.name}/flit_core";
 
   buildPhase = ''
     runHook preBuild