Add `dependencies` to find transitive dependencies

[ericphanson]

Based on #27

[oxinabox]

I wonder if we should have a max_depth argument? defaulting to type_max(Int)

[ericphanson]

I wonder if we should have a max_depth argument? defaulting to type_max(Int)

We don't need to worry about cycles because if we find a package twice we don't recurse the second time, in case that's the concern. And it's pretty fast:

julia> @time dependencies("DifferentialEquations")
  0.055740 seconds (166.06 k allocations: 14.614 MiB)
Dict{String, Base.UUID} with 157 entries:
  "Pkg"                    => UUID("44cfe95a-1eb2-52ea-b672-e2afdf69b78f")
  "ForwardDiff"            => UUID("f6369f11-7733-5829-9624-2563aa707210")
  "RecursiveFactorization" => UUID("f2c3362d-daeb-58d1-803e-2bc74f2840b4")
  "SuiteSparse"            => UUID("4607b0f0-06f3-5cda-b6b1-a6196a1729e9")
  "ParameterizedFunctions" => UUID("65888b18-ceab-5e60-b2b9-181511a3b968")
  "DelayDiffEq"            => UUID("bcd4f6db-9728-5f36-b5f7-82caef46ccdb")
  "MuladdMacro"            => UUID("46d2c3a1-f734-5fdb-9937-b9b9aeba4221")
  "TableTraits"            => UUID("3783bdb8-4a98-5b6b-af9a-565f29a5fe9c")
  "OffsetArrays"           => UUID("6fe1bfb0-de20-5000-8ca7-80f57d26f881")
  "ConstructionBase"       => UUID("187b0558-2788-49d3-abe0-74a17ed4e7c9")
  "OrderedCollections"     => UUID("bac558e1-5e72-5ebc-8fee-abe8a469f55d")
  "MultiScaleArrays"       => UUID("f9640e96-87f6-5992-9c3b-0743c6a49ffa")
  "StochasticDiffEq"       => UUID("789caeaf-c7a9-5a7d-9973-96adeb23e2a0")
  ⋮                        => ⋮

So I don't really see the need for limiting the depth for a perf point of view. Is it to use as a feature to see like level-1 deps, level-2 deps, etc?

[mattBrzezinski]

LGTM!

[ericphanson]

Thanks Matt! Then I'll leave the max-depth as a possible future feature in the interest of getting this in. Could you take a quick look at #27? This PR is pointing at that one, so I'd rather merge 27 into master and then this into master, rather than merging this into 27.