Fix and test derivative(x, 2x)

[LilithHafner]

Fixes #1077

[ChrisRackauckas]

It's a unityper type https://github.com/YingboMa/Unityper.jl

https://github.com/JuliaSymbolics/SymbolicUtils.jl/blob/d68e092a646ee746becc16fa5ec0e7e066a96f12/src/types.jl#L25-L62

So you need to check the EXPR type https://github.com/JuliaSymbolics/SymbolicUtils.jl/blob/d68e092a646ee746becc16fa5ec0e7e066a96f12/src/types.jl#L252

[LilithHafner]

This breaks Symbolics.derivative(2x, 2x) == 1. IDK if we care about that.

[ChrisRackauckas]

I am fine with having more errors, more safety.

[LilithHafner]

This also breaks differentiating with respect to an array of symbols (which we should presumably fix) and with respect to x(t) which occurs once in tests.

[LilithHafner]

The crux of this issue is that we assume two expressions are constant with respect to eachother unless proven otherwise.

julia> @variables x, y

julia> Symbolics.derivative(x, y)
0

This PR does not fix that issue, and does introduce errors which previously worked e.g. all of these results will become errors:

julia> @variables x(t), y(t)

julia> Symbolics.derivative(x, x) # Good
1

julia> Symbolics.derivative(t, x) # Wrong
0

julia> Symbolics.derivative(x, t) # Good
Differential(t)(x(t))

[YingboMa]

We shouldn't limit it to BasicSymbolic

[YingboMa]

Should just check !istree

[YingboMa]

This PR uses unnecessary internal details.

[YingboMa]

This is also not quite correct. We frequently need to differentiate wrt variables with an independent variable like x(t).