Pre-allocate seen to improve inference in unique(f, itr)

JuliaLang/julia#36280 introduced the ability to
pre-allocate the container used to track values of `f.(itr)` in
`unique(f, itr)`. Particularly for containers with `Union` elements,
this circumvents significant inference problems.

Related: JuliaLang/julia#36454

src/Registry.jl

@@ -3,7 +3,7 @@ module Registry
 import ..Pkg, ..Types, ..API
 using ..Pkg: depots1
 using ..Types: RegistrySpec, Context, Context!
-
+using UUIDs
 
 """
     Pkg.Registry.add(url::String)
@@ -103,7 +103,7 @@ status(; kwargs...) = status(Context(); kwargs...)
 function status(ctx::Context; io::IO=stdout, as_api=false, kwargs...) # TODO split as_api into own function
     Context!(ctx; io=io, kwargs...)
     regs = Types.collect_registries()
-    regs = unique(r -> r.uuid, regs) # Maybe not?
+    regs = unique(r -> r.uuid, regs; seen=Set{Union{UUID,Nothing}}()) # Maybe not?
     as_api && return regs
     Types.printpkgstyle(ctx, Symbol("Registry Status"), "")
     if isempty(regs)

src/Types.jl

@@ -1109,7 +1109,7 @@ function update_registries(ctx::Context, regs::Vector{RegistrySpec} = collect_re
     !force && UPDATED_REGISTRY_THIS_SESSION[] && return
     errors = Tuple{String, String}[]
     registry_urls = nothing
-    for reg in unique(r -> r.uuid, find_installed_registries(ctx, regs))
+    for reg in unique(r -> r.uuid, find_installed_registries(ctx, regs); seen=Set{Union{UUID,Nothing}}())
         let reg=reg
             regpath = pathrepr(reg.path)
             if isfile(joinpath(reg.path, ".tree_info.toml"))