[BUG] Backend init broken in Julia 1.10 if loaded during precompilation

[BioTurboNick]

https://github.com/JuliaPlots/Plots.jl/blob/0d956739d57d87b7557649aaf4948fcf24fb60d4/src/backends.jl#L396C1-L409C4

function _initialize_backend(pkg::AbstractBackend)
    _pre_imports(pkg)
    name = backend_package_name(pkg)
    # NOTE: this is a hack importing in `Main` (expecting the package to be in `Project.toml`, remove in `[email protected]`)
    # FIXME: remove hard `GR` dependency in `[email protected]`
    @eval name === :GR ? Plots : Main begin
        import $name
        export $name
        $(_check_compat)($name)
    end
    _post_imports(pkg)
    _runtime_init(pkg)
    nothing
end

Causes issues like this in CI: https://github.com/JuliaPlots/Plots.jl/actions/runs/7443223127/job/20247799581

ERROR: LoadError: Creating a new global in closed module `Main` (`UnicodePlots`) breaks incremental compilation because the side effects will not be permanent.
Stacktrace:
  [1] top-level scope
    @ ~/work/Plots.jl/Plots.jl/src/backends.jl:402
  [2] eval
    @ Core ./boot.jl:385 [inlined]
  [3] _initialize_backend(pkg::Plots.UnicodePlotsBackend)
    @ Plots ~/work/Plots.jl/Plots.jl/src/backends.jl:401
  [4] backend(pkg::Plots.UnicodePlotsBackend)
    @ Plots ~/work/Plots.jl/Plots.jl/src/backends.jl:266

But can also occur in other scenarios: JuliaPlots/PlotlyJS.jl#478

[BioTurboNick]

Guess this was known, a shame it hasn't been addressed: JuliaLang/julia#51117

[BeastyBlacksmith]

I think the course of action here, is to release a new point version that removes the precompilation of non-GR default backends on 1.10+ and then go full package extensions in 2.0 ( xref: #4565 ).