Simplify solve_docp core

[ocots]

@PierreMartinon We could replace

CTDirect.jl/src/solve.jl

Lines 131 to 143 in 5a2105a

	abstract type SolverTag end
	struct IpoptTag <: SolverTag end
	struct MadNLPTag <: SolverTag end
	function solve_docp(solver_tag, args...; kwargs...)
	if typeof(solver_tag) == IpoptTag
	throw(ExtensionError(:NLPModelsIpopt))
	elseif typeof(solver_tag) == MadNLPTag
	throw(ExtensionError(:MadNLP))
	else
	error("Unknown solver type", typeof(solver_tag))
	end
	end

by

abstract type AbstractSolverBackend end
struct IpoptBackend <: AbstractSolverBackend end
struct MadNLPBackend <: AbstractSolverBackend end

weakdeps = Dict(IpoptBackend => :NLPModelsIpopt, MadNLPBackend => :MadNLP)

function solve_docp(solver_tag::T, args...; kwargs...) where {T <: AbstractSolverBackend}
    throw(ExtensionError(weakdeps[T]))
end

[PierreMartinon]

@PierreMartinon We could replace

CTDirect.jl/src/solve.jl

Lines 131 to 143 in 5a2105a

	abstract type SolverTag end
	struct IpoptTag <: SolverTag end
	struct MadNLPTag <: SolverTag end
	function solve_docp(solver_tag, args...; kwargs...)
	if typeof(solver_tag) == IpoptTag
	throw(ExtensionError(:NLPModelsIpopt))
	elseif typeof(solver_tag) == MadNLPTag
	throw(ExtensionError(:MadNLP))
	else
	error("Unknown solver type", typeof(solver_tag))
	end
	end

by

abstract type AbstractSolverBackend end
struct IpoptBackend <: AbstractSolverBackend end
struct MadNLPBackend <: AbstractSolverBackend end

weakdeps = Dict(IpoptBackend => :NLPModelsIpopt, MadNLPBackend => :MadNLP)

function solve_docp(solver_tag::T, args...; kwargs...) where {T <: AbstractSolverBackend}
    throw(ExtensionError(weakdeps[T]))
end

This looks so neat and classy OO