Revert HGTD identifier levels (for now) to those used in 20.20

This MR reverts the identifier levels for HGTD from the new [endcap, disk, side, quadrant, row, module] back to [endcap, layer, phi_module, eta_module] which is essentially what was used in 20.20.

While in the former case the levels better exploit the symmetries of the HGTD and should be implemented for the longer term (migration to master?), the levels which divide the modules by phi/eta will allow for a quicker integration of HGTD sim/digi/reco into 21.9 due to (among other reasons) a more out-of-the-box usage of SiHits for HGTD simulation.

Eta/phi neighbor methods in HGTD_ID are also re-instated in this update.