`fix` and `condition` _still_ don't use the same VarNames as tilde pipeline (specifically InitFromParams)

[penelopeysm]

This is a resurfacing of #702:

julia> using DynamicPPL, Distributions, LinearAlgebra

julia> @model function f()
           x ~ MvNormal(zeros(2), I)
           return x
       end
f (generic function with 2 methods)

julia> first(DynamicPPL.init!!(f(), VarInfo(), InitFromParams(Dict(@varname(x[1]) => 1.0, @varname(x[2]) => 2.0))))
2-element Vector{Float64}:
 1.0
 2.0

julia> fix(f(), Dict(@varname(x[1]) => 1.0, @varname(x[2]) => 2.0))()
2-element Vector{Float64}:
 0.24589494968073047
 1.0916208950180246

I think that the current behaviour is quite dangerous because it silently fails rather than erroring. In particular, the main failure mode would be attempting to use a Dict obtained from MCMCChains.

The reason for this difference in behaviour is because hasfixed only uses hasvalue(context.values, vn), instead of using hasvalue(context.values, vn, dist). This is a fairly easy fix, we just need to pass dist down through the call stack of isfixed, etc.

Because hasvalue(vals, vn, dist) simply calls hasvalue(vals, vn) as the very first check, I don't believe this shouldn't result in any (significant) loss of performance for the existing case where the whole vector x is provided. But it'd probably be good to benchmark in case.