Manifest with a dev-ed package that has sources entry internal errors if source is missing

[IanButterworth]

If your manifest states a package is dev-ed at a specific path, but it's missing, then instantiate fails with this internal error.

I'm not sure what it should do in this case, where it has a [sources] entry, so it knows where it should come from.

[[deps.BaseBenchmarks]]
deps = ["BenchmarkTools", "Dates", "Distributed", "InteractiveUtils", "LinearAlgebra", "Printf", "REPL", "Random", "Serialization", "SparseArrays", "StableRNGs", "Statistics"]
path = "~/.julia/dev/BaseBenchmarks"
uuid = "d7f09723-0a21-57e2-b9ef-316b714b6879"
version = "0.0.0"

(testenvs) pkg> st
Status `~/Documents/GitHub/Nanosoldier.jl/testenvs/Project.toml`
→ [d7f09723] BaseBenchmarks v0.0.0 `https://github.com/JuliaCI/BaseBenchmarks.jl#master` `~/.julia/dev/BaseBenchmarks`
  [6e4b80f9] BenchmarkTools v1.7.0
  [682c06a0] JSON v1.5.0
Info Packages marked with → are not downloaded, use `instantiate` to download

(testenvs) pkg> instantiate
ERROR: MethodError: no method matching joinpath(::Nothing)
The function `joinpath` exists, but no method is defined for this combination of argument types.

Closest candidates are:
  joinpath(::Base.Filesystem.DirEntry, Any...)
   @ Base file.jl:1064
  joinpath(::AbstractString...)
   @ Base path.jl:487
  joinpath(::Union{Tuple, AbstractVector})
   @ Base path.jl:465

Stacktrace:
  [1] stat(path::Nothing)
    @ Base.Filesystem ./stat.jl:235
  [2] isdir(path::Nothing)
    @ Base.Filesystem ./stat.jl:587
  [3] instantiate(ctx::Pkg.Types.Context; manifest::Nothing, update_registry::Bool, verbose::Bool, platform::Base.BinaryPlatforms.Platform, allow_build::Bool, allow_autoprecomp::Bool, workspace::Bool, julia_version_strict::Bool, kwargs::@Kwargs{})
    @ Pkg.API ~/.julia/juliaup/julia-nightly/Julia-1.14.app/Contents/Resources/julia/share/julia/stdlib/v1.14/Pkg/src/API.jl:1376
  [4] instantiate
    @ ~/.julia/juliaup/julia-nightly/Julia-1.14.app/Contents/Resources/julia/share/julia/stdlib/v1.14/Pkg/src/API.jl:1292 [inlined]
  [5] instantiate()
    @ Pkg.API ~/.julia/juliaup/julia-nightly/Julia-1.14.app/Contents/Resources/julia/share/julia/stdlib/v1.14/Pkg/src/API.jl:1291
  [6] do_cmd(command::Pkg.REPLMode.Command, io::Base.TTY)
    @ Pkg.REPLMode ~/.julia/juliaup/julia-nightly/Julia-1.14.app/Contents/Resources/julia/share/julia/stdlib/v1.14/Pkg/src/REPLMode/REPLMode.jl:418
  [7] do_cmds(commands::Vector{Pkg.REPLMode.Command}, io::Base.TTY)
    @ Pkg.REPLMode ~/.julia/juliaup/julia-nightly/Julia-1.14.app/Contents/Resources/julia/share/julia/stdlib/v1.14/Pkg/src/REPLMode/REPLMode.jl:405
  [8] do_cmds(repl::REPL.LineEditREPL, commands::String)
    @ REPLExt ~/.julia/juliaup/julia-nightly/Julia-1.14.app/Contents/Resources/julia/share/julia/stdlib/v1.14/Pkg/ext/REPLExt/REPLExt.jl:125
  [9] on_done(s::REPL.LineEdit.MIState, buf::IOBuffer, ok::Bool, repl::REPL.LineEditREPL)
    @ REPLExt ~/.julia/juliaup/julia-nightly/Julia-1.14.app/Contents/Resources/julia/share/julia/stdlib/v1.14/Pkg/ext/REPLExt/REPLExt.jl:139
 [10] #invokelatest_gr#224
    @ ./reflection.jl:1372 [inlined]
 [11] invokelatest_gr
    @ ./reflection.jl:1366 [inlined]
 [12] (::REPLExt.var"#create_mode##0#create_mode##1"{REPL.LineEditREPL})(s::REPL.LineEdit.MIState, buf::IOBuffer, ok::Bool)
    @ REPLExt ~/.julia/juliaup/julia-nightly/Julia-1.14.app/Contents/Resources/julia/share/julia/stdlib/v1.14/Pkg/ext/REPLExt/REPLExt.jl:165
 [13] run_interface(terminal::Base.Terminals.TextTerminal, m::REPL.LineEdit.ModalInterface, s::REPL.LineEdit.MIState)
    @ REPL.LineEdit ~/.julia/juliaup/julia-nightly/Julia-1.14.app/Contents/Resources/julia/share/julia/stdlib/v1.14/REPL/src/LineEdit.jl:2945
 [14] run_frontend(repl::REPL.LineEditREPL, backend::REPL.REPLBackendRef)
    @ REPL ~/.julia/juliaup/julia-nightly/Julia-1.14.app/Contents/Resources/julia/share/julia/stdlib/v1.14/REPL/src/REPL.jl:1697
 [15] (::REPL.var"#62#63"{REPL.LineEditREPL, REPL.REPLBackendRef})()
    @ REPL ~/.julia/juliaup/julia-nightly/Julia-1.14.app/Contents/Resources/julia/share/julia/stdlib/v1.14/REPL/src/REPL.jl:688