[FR] redesign backend dependencies

[t-bltg]

Some nice design ideas by @mkitti in #4566 (review):

Perhaps we should consider a more radical redesign in the near future?

1. Each of the pieces of backend glue code could be independent subpackages. GRPlotsBackend, PyPlotPlotsBackend, etc.
2. Most of the code currently in Plots.jl gets moved to a package named AbstractPlots.jl. All the backend packages depend on AbstractPlots.jl.
3. Plots.jl becomes a small package which just depends on GRPlotsBackend.jl by default. Via Preferences.jl it can optionally load any of the other backend packages and act as a proxy for that package.

The advantage of this is that each backend now can be precompiled and has its own compiled cache. Also the project dependency tree becomes a bit clearer. One could get all the features of Plots by just doing using SomePlotsBackend.jl directly.

For loading with Requires.jl can then just do @eval using GRPlotsBackend.jl similar to what @timholy recommended here:
https://discourse.julialang.org/t/categoricalarrays-forces-openspecfun-jll-recompilation/87759/18?u=mkitti

The impact here for this PR is still acknowledging that the GR backend is a special case for the current architecture. That is Plots.jl has declared a dependency on GR.jl. One consequence of that special case is that we load GR.jl within Plots.jl rather than Main. Eventually we move that responsibility to GRPlotsBackend.jl.

JuliaLang/julia/pull/47040 or JuliaLang/julia/pull/47695 could help too.

[BeastyBlacksmith]

1. Each of the pieces of backend glue code could be independent subpackages. GRPlotsBackend, PyPlotPlotsBackend, etc.

Back in the days (6 years ago) that was how things were done, e.g. https://github.com/JuliaPlots/PlotsGR.jl

[t-bltg]

Fixing #4589 would improve the current.

[t-bltg]

Implemented in #4904.