Test errors on DynamicPPL due to Libtask

[devmotion]

CI tests of DynamicPPL (see https://travis-ci.com/github/TuringLang/DynamicPPL.jl/jobs/306791553 and TuringLang/DynamicPPL.jl#52) fail on Julia 1.4 and Julia master due to the error

ERROR: LoadError: The following package names could not be resolved:
 * Libtask (not found in project or manifest)
Stacktrace:
 [1] pkgerror(::String) at /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.4/Pkg/src/Types.jl:53
 [2] ensure_resolved(::Pkg.Types.Context, ::Array{Pkg.Types.PackageSpec,1}; registry::Bool) at /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.4/Pkg/src/Types.jl:794
 [3] ensure_resolved at /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.4/Pkg/src/Types.jl:759 [inlined]
 [4] build(::Pkg.Types.Context, ::Array{Pkg.Types.PackageSpec,1}; verbose::Bool, kwargs::Base.Iterators.Pairs{Union{},Union{},Tuple{},NamedTuple{(),Tuple{}}}) at /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.4/Pkg/src/API.jl:697
 [5] build at /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.4/Pkg/src/API.jl:682 [inlined]
 [6] #build#108 at /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.4/Pkg/src/API.jl:680 [inlined]
 [7] build at /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.4/Pkg/src/API.jl:680 [inlined]
 [8] #build#105 at /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.4/Pkg/src/API.jl:677 [inlined]
 [9] build(::String) at /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.4/Pkg/src/API.jl:677
 [10] top-level scope at /home/travis/.julia/packages/Libtask/K7OAU/src/Libtask.jl:7
 [11] include(::Module, ::String) at ./Base.jl:377
 [12] top-level scope at none:2
 [13] eval at ./boot.jl:331 [inlined]
 [14] eval(::Expr) at ./client.jl:449
 [15] top-level scope at ./none:3
in expression starting at /home/travis/.julia/packages/Libtask/K7OAU/src/Libtask.jl:5

I think the problematic lines are

Libtask.jl/src/Libtask.jl

Lines 5 to 8 in a1ce1ea

	if !isfile(joinpath((@__DIR__) |> dirname, "deps/desp.jl"))
	import Pkg
	Pkg.build("Libtask")
	end

Can we remove them completely? I would assume that Pkg should run Pkg.build() automatically?

[devmotion]

BTW I guess it's a typo and should actually be "deps/deps.jl" in line 5?