feat: Make Cargo.toml source locations explicit

This allows cargo metadata to operate on it without adding the
source files to the build. (A choice which will save a few rebuilds
of the manual)

(cherry picked from commit 1779295f3e13cc15f8422d52a3753bb927ac8fa7)

rust/nix-c-raw/Cargo.toml

@@ -5,6 +5,7 @@ edition = "2021"
 build = "build.rs"
 
 [lib]
+path = "src/lib.rs"
 
 [build-dependencies]
 bindgen = "0.69.4"

rust/nix-expr/Cargo.toml

@@ -4,6 +4,7 @@ version = "0.1.0"
 edition = "2021"
 
 [lib]
+path = "src/lib.rs"
 
 [dependencies]
 anyhow = "1.0.79"

rust/nix-store/Cargo.toml

@@ -4,6 +4,7 @@ version = "0.1.0"
 edition = "2021"
 
 [lib]
+path = "src/lib.rs"
 
 [dependencies]
 anyhow = "1.0.79"

rust/nix-util/Cargo.toml

@@ -4,6 +4,7 @@ version = "0.1.0"
 edition = "2021"
 
 [lib]
+path = "src/lib.rs"
 
 [dependencies]
 anyhow = "1.0.79"