Put subfiles into subdirectory with the package name

This allows someone to use `\include lilydrum/embellishments.ily` if needed.

lilydrum.ly

@@ -26,12 +26,12 @@
 
 #(ly:set-option 'relative-includes #t)
 
-\include "lib/internal_functions.ily"
+\include "lilydrum/internal_functions.ily"
 
-\include "lib/layout_tweaks.ily"
-\include "lib/musical_functions.ily"
-\include "lib/embellishments.ily"
-\include "lib/flourishing.ily"
-\include "lib/special_sticking.ily"
-\include "lib/rehearsal_marks.ily"
-\include "lib/midi.ily"
+\include "lilydrum/layout_tweaks.ily"
+\include "lilydrum/musical_functions.ily"
+\include "lilydrum/embellishments.ily"
+\include "lilydrum/flourishing.ily"
+\include "lilydrum/special_sticking.ily"
+\include "lilydrum/rehearsal_marks.ily"
+\include "lilydrum/midi.ily"