Switch datastructure Vector -> Set for algorithmic complexity (#3652)

In the same project as #3651, this code was also having trouble
because of the rapidly growing alldeps, which would end up with
many repeated entries. Switch the data structure to fix the algorithmic
complexity here also.

(cherry picked from commit 0e0cf45)

src/API.jl

@@ -1248,19 +1248,21 @@ function precompile(ctx::Context, pkgs::Vector{PackageSpec}; internal_call::Bool
     # if a list of packages is given, restrict to dependencies of given packages
     if !isempty(pkgs)
         pkgs_names = [p.name for p in pkgs]
-        function collect_all_deps(depsmap, dep, alldeps=Base.PkgId[])
-            append!(alldeps, depsmap[dep])
+        function collect_all_deps(depsmap, dep, alldeps=Set{Base.PkgId}())
             for _dep in depsmap[dep]
-                collect_all_deps(depsmap, _dep, alldeps)
+                if !(_dep in alldeps)
+                    push!(alldeps, _dep)
+                    collect_all_deps(depsmap, _dep, alldeps)
+                end
             end
             return alldeps
         end
-        keep = Base.PkgId[]
+        keep = Set{Base.PkgId}()
         for dep in depsmap
             dep_pkgid = first(dep)
             if dep_pkgid.name in pkgs_names
                 push!(keep, dep_pkgid)
-                append!(keep, collect_all_deps(depsmap, dep_pkgid))
+                collect_all_deps(depsmap, dep_pkgid, keep)
             end
         end
         for ext in keys(exts)