Fix clobbering with multi-backend packaging

[AmintorDusko]

Context: Installing multi-backends clobbers the common core package. This PR allows for sensible install/uninstall of multiple backends.

Description of the Change: Here we make the PennyLane_Lightning package (lightning.qubit) responsible for the core package. Any other package (backends) will require PennyLane_Lightning (same version) from now on.

Benefits: Better and sensible packaging

Possible Drawbacks:

Related GitHub Issues:

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.04% 🎉

Comparison is base (67aaaf6) 99.08% compared to head (0e94cab) 99.13%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #504      +/-   ##
==========================================
+ Coverage   99.08%   99.13%   +0.04%     
==========================================
  Files         142      142              
  Lines       16798    16798              
==========================================
+ Hits        16644    16652       +8     
+ Misses        154      146       -8     

Files Changed	Coverage Δ
pennylane_lightning/core/_version.py	100.00% <100.00%> (ø)

... and 3 files with indirect coverage changes

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[mlxd]

Thanks @AmintorDusko

I think we can opt for this now, and refine the file/package ownerships once we have all 3 devices integrated.

[vincentmr]

Looks good to me, thanks @AmintorDusko .