PluggableUSB.h location inconsistency

AVR arduino has PluggableUSB.h in 'arduino' subfolder, while the ARM based cores (SAMD including) have it in 'USB' subfolder, making it difficult to have a library which supports both cores.

You may fix this inconsistency by placing a 'dummy' PluggableUSB.h in the ARM cores in arduino/ subfolder which simply includes the USB/PluggableUSB.h, or do the reverse of this in the AVR core.

Compare the AVR core's arduino/PluggableUSB.h with the ARM core's USB/PluggableUSB.h, then inspect the corresponding include paths in this repository. Choose the compatibility arrangement described in the issue and verify that a library can use the same PluggableUSB.h include path for both AVR and ARM cores.

Written by the indexing model from the issue text.