Shift cargo mutants inclusion files to be within the core module

After payjoin#746 was merged a small path discrepency was created with the core
module now being the direct path to all the src modules. This Pr simply
adds core/ into the path so that cargo mutants has the correct path to
find those modules.

Author: benalleng

.cargo/mutants.toml

@@ -1,5 +1,5 @@
 additional_cargo_args = ["--all-features"]
-examine_globs = ["payjoin/src/uri/*.rs", "payjoin/src/receive/**/*.rs", "payjoin/src/send/**/*.rs"]
+examine_globs = ["payjoin/src/core/uri/*.rs", "payjoin/src/core/receive/**/*.rs", "payjoin/src/core/send/**/*.rs"]
 exclude_globs = []
 exclude_re = [
 	"impl Debug",