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/grouping/mod.rs

@@ -0,0 +1,17 @@
+/*!
+This module reflects the relationship between groups, sections / backends and
+packages.
+
+A [`Group`] contains one (strictly spoken zero, but this doesn't make sense) or
+more [`Section`]s, which relate to individual backends. Each section contains
+one (strictly spoken zero) or more [`Package`]s. On start-up `pacdef` will load
+all groups using [`Group::load`], which in turn will get all packages from all
+sections.
+*/
+mod group;
+mod package;
+mod section;
+
+pub use group::Group;
+pub use package::Package;
+pub use section::Section;