Get path to installed extensions.

[fractaledmind]

In order for this gem to work seamlessly with new features added to the sqlite3-ruby gem (see sparklemotion/sqlite3-ruby#586) and rails (see rails/rails#53827) that make loading extensions easier, this gem needs to provide Ruby constants for any and all installed extensions that respond to the .to_path method.

Since only installed extensions should have constants, constants need to be dynamically defined. This PR adds a const_missing hook to dynamically resolve extension constants for installed extensions.

cc @flavorjones @rossta

[fractaledmind]

Will be working on tests here later today. That is the last piece before merging and cutting a new release.

[flavorjones]

If the constants resolution gets hairy, it might still be a solid step forward to have the gem provide a method that returns a path, for use in ERB blocks:

production:
  adapter: sqlite2
  database: storage/production.sqlite3
  extensions:
    - <%= Sqlpkg.path_for("nalgeon/crypto") %>

[fractaledmind]

@flavorjones Good point. This is simpler and much less error prone. Going with this for now.