restructure project into a virtual manifest

- remove redundant readme
- de-duplicate Cargo.toml information using workspace inheritance
- rename "pacdef_core" to "pacdef"
- move "crates/main/main.rs" to "crates/pacdef/src/main.rs"

crates/pacdef/build.rs

@@ -0,0 +1,13 @@
+use std::process::Command;
+
+fn main() {
+    let git_hash = match Command::new("git")
+        .args(["rev-parse", "--short", "HEAD"])
+        .output()
+    {
+        Ok(output) => String::from_utf8(output.stdout).expect("git output is utf-8"),
+        _ => String::new(),
+    };
+
+    println!("cargo:rustc-env=GIT_HASH={git_hash}");
+}