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/src/backend/actual/rust.rs

@@ -0,0 +1,91 @@
+use std::collections::HashSet;
+use std::fs::read_to_string;
+use std::io::ErrorKind::NotFound;
+use std::path::PathBuf;
+
+use anyhow::{bail, Context, Result};
+use serde_json::Value;
+
+use crate::backend::backend_trait::{Backend, Switches, Text};
+use crate::backend::macros::impl_backend_constants;
+use crate::{Group, Package};
+
+#[derive(Debug, Clone)]
+pub struct Rust {
+    pub(crate) packages: HashSet<Package>,
+}
+
+const BINARY: Text = "cargo";
+const SECTION: Text = "rust";
+
+const SWITCHES_INSTALL: Switches = &["install"];
+const SWITCHES_INFO: Switches = &["search", "--limit", "1"];
+const SWITCHES_MAKE_DEPENDENCY: Switches = &[];
+const SWITCHES_NOCONFIRM: Switches = &[]; // not needed
+const SWITCHES_REMOVE: Switches = &["uninstall"];
+
+const SUPPORTS_AS_DEPENDENCY: bool = false;
+
+impl Backend for Rust {
+    impl_backend_constants!();
+
+    fn get_all_installed_packages(&self) -> Result<HashSet<Package>> {
+        let file = get_crates_file().context("getting path to crates file")?;
+
+        let content = match read_to_string(file) {
+            Ok(string) => string,
+            Err(err) if err.kind() == NotFound => {
+                eprintln!(
+                    "WARNING: no crates file found for cargo. Assuming no crates installed yet."
+                );
+                return Ok(HashSet::new());
+            }
+            Err(err) => bail!(err),
+        };
+
+        let json: Value =
+            serde_json::from_str(&content).context("parsing JSON from crates file")?;
+        extract_packages(&json).context("extracing packages from crates file")
+    }
+
+    fn get_explicitly_installed_packages(&self) -> Result<HashSet<Package>> {
+        self.get_all_installed_packages()
+            .context("getting all installed packages")
+    }
+
+    fn make_dependency(&self, _: &[Package]) -> Result<()> {
+        panic!("not supported by {}", BINARY)
+    }
+}
+
+fn extract_packages(json: &Value) -> Result<HashSet<Package>> {
+    let result: HashSet<_> = json
+        .get("installs")
+        .context("get 'installs' field from json")?
+        .as_object()
+        .context("getting object")?
+        .into_iter()
+        .map(|(name, _)| name)
+        .map(|name| {
+            name.split_whitespace()
+                .next()
+                .expect("identifier is whitespace-delimited")
+        })
+        .map(|name| Package::try_from(name).expect("name is valid"))
+        .collect();
+    Ok(result)
+}
+
+impl Rust {
+    pub(crate) fn new() -> Self {
+        Self {
+            packages: HashSet::new(),
+        }
+    }
+}
+
+fn get_crates_file() -> Result<PathBuf> {
+    let mut result = crate::path::get_cargo_home().context("getting cargo home dir")?;
+    result.push(".crates2.json");
+    Ok(result)
+}