Core_ Update package name

[Kimhaedam]

The package of euphony is co.euphony, while the package of unit test is euphony.lib.
So I want to fit this consistently!

TO DO

• Change the package on the unit test side.
  • euphony.lib -> co.euphony

[yeonns]

@Kimhaedam Thank you for suggestion! With this update, we will have consistency 👍