Prefer require_relative for internal requires

`require_relative` is preferred over `require` for files within the same
project because it uses paths relative to the current file, making code
more portable and less dependent on the load path.

This change updates internal requires to use `require_relative` for
consistency, performance, and improved portability.

Ref:
- ruby/psych#522
- ruby/logger#20
- ruby/rdoc#658
- #349
- rubocop/rubocop#8748

lib/memo_wise.rb

@@ -2,8 +2,8 @@
 
 require "set" # Ruby < 3.2 does not load `set` by default.
 
-require "memo_wise/internal_api"
-require "memo_wise/version"
+require_relative "memo_wise/internal_api"
+require_relative "memo_wise/version"
 
 # MemoWise is the wise choice for memoization in Ruby.
 #