Precompilation: Try adding failsafe against deadlocks

[IanButterworth]

As discussed in #2057 it'd be good if deadlocks weren't possible.

This does two things:

• Assert that all deps of the entries in a manifest are themselves entries in the Manifest (if not, that would've deadlocked)
• As a more general unknown deadlock avoidance, allow each task to skip waiting for a dep if the precomp tasks have been idle for >~250ms

[IanButterworth]

With this PR, was_processed Events are just being used for their underlying Bool, so I just switched it to a Bool

[IanButterworth]

I tried deleting an ColorTypes from the env Manifest that another package depended on, and it was caught by Context(), so the @assert isn't needed

julia> Pkg.precompile()
ERROR: `ColorVectorSpace=c3611d14-8923-5661-9e6a-0046d554d3a4` depends on `ColorTypes`, but no such entry exists in the manifest.
Stacktrace:
  [1] pkgerror(::String, ::Vararg{String, N} where N)
    @ Pkg.Types ~/Documents/GitHub/Pkg.jl/src/Types.jl:52
  [2] normalize_deps(name::String, uuid::Base.UUID, deps::Vector{String}, manifest::Dict{String, Vector{Pkg.Types.Stage1}})
    @ Pkg.Types ~/Documents/GitHub/Pkg.jl/src/manifest.jl:95
  [3] validate_manifest(stage1::Dict{String, Vector{Pkg.Types.Stage1}})
    @ Pkg.Types ~/Documents/GitHub/Pkg.jl/src/manifest.jl:109
  [4] Dict{Base.UUID, Pkg.Types.PackageEntry}(raw::Dict{String, Any})
    @ Pkg.Types ~/Documents/GitHub/Pkg.jl/src/manifest.jl:158
  [5] read_manifest(f_or_io::String)
    @ Pkg.Types ~/Documents/GitHub/Pkg.jl/src/manifest.jl:170
  [6] Pkg.Types.EnvCache(env::Nothing)
    @ Pkg.Types ~/Documents/GitHub/Pkg.jl/src/Types.jl:294
  [7] EnvCache
    @ ~/Documents/GitHub/Pkg.jl/src/Types.jl:274 [inlined]
  [8] Pkg.Types.Context()
    @ Pkg.Types ./util.jl:442
  [9] #precompile#195
    @ ~/Documents/GitHub/Pkg.jl/src/API.jl:913 [inlined]
 [10] precompile
    @ ~/Documents/GitHub/Pkg.jl/src/API.jl:913 [inlined]

I'm also going off the approach in this PR.. it does away with a fair amount of elegance, and increases the no-op about 10-20%..
I'm going to mark this draft and try to figure out how the #2057 deadlock is happening..

[IanButterworth]

Not needed anymore. precompile guards against circular deps, and we haven't seen any other deadlocks being reported