Replace wpi_interface with wpilib_interface

[aidnem]

The file structure contains a folder: wpi_interface, but the java source files themselves describe themselves as being part of package coppercore.wpilib_interface.

Use wpilib_interface for everything, all tickets on wpi_interface should rename it to wpilib_interface before they are merged!

[aidnem]

@linglejack06 @avidraccoon please be sure to rename wpi_interface to wpilib_interface in the files and in the settings.gradle before merging your PRs.

[jkleiber]

done in #36