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/errors.rs

@@ -0,0 +1,51 @@
+use std::error::Error as ErrorTrait;
+use std::fmt::{Display, Write};
+use std::path::PathBuf;
+
+/// Error types for pacdef.
+#[derive(Debug)]
+#[non_exhaustive]
+pub enum Error {
+    /// Package search yields no results.
+    NoPackagesFound,
+    /// Config file not found.
+    ConfigFileNotFound,
+    /// Group file not found.
+    GroupFileNotFound(String),
+    /// Group already exists.
+    GroupAlreadyExists(PathBuf),
+    /// Invalid group name ('.' or '..')
+    InvalidGroupName(String),
+    /// Multiple groups not found.
+    MultipleGroupsNotFound(Vec<String>),
+}
+
+impl Display for Error {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            Self::NoPackagesFound => f.write_str("no packages matching query"),
+            Self::ConfigFileNotFound => f.write_str("config file not found"),
+            Self::GroupFileNotFound(name) => f.write_str(&format!("group file '{name}' not found")),
+            Self::GroupAlreadyExists(path) => f.write_str(&format!(
+                "group file '{}' already exists",
+                path.to_string_lossy()
+            )),
+            Self::InvalidGroupName(name) => {
+                f.write_str(&format!("group name '{name}' is not valid"))
+            }
+            Self::MultipleGroupsNotFound(vec) => {
+                f.write_str("could not find the following groups:\n")?;
+                let mut iter = vec.iter().peekable();
+                while let Some(group) = iter.next() {
+                    f.write_str(&format!("  {group}"))?;
+                    if iter.peek().is_some() {
+                        f.write_char('\n')?;
+                    }
+                }
+                Ok(())
+            }
+        }
+    }
+}
+
+impl ErrorTrait for Error {}