feat(groups): allow nested group dirs

Instead of expecting all files to be located immediately under the group
dir, we allow them to be nested in additional folders. We walk the group
folder, and treat every file as a group file. The relative path of the
file as seen from the group dir becomes the group name.

Fixes #27.

crates/pacdef_core/Cargo.toml

@@ -19,6 +19,7 @@ const_format = { version = "0.2", default-features = false }
 path-absolutize = "3.0"
 regex = { version = "1.7", default-features = false, features = ["std"] }
 termios = "0.3"
+walkdir = "2.3"
 
 serde = "1.0"
 serde_derive = "1.0"