Try adding support for testing downstream subdir packages

Author: mtfishman

.github/workflows/IntegrationTest.yml

@@ -54,11 +54,20 @@ jobs:
             end
           end
           try
-            Pkg.develop(PackageSpec(path="downstream"))
+            repo_and_subdir = split("${{ inputs.repo }}", ':')
+            repo = first(repo_and_subdir)
+            pkg_name = string(first(split(last(split(repo, '/')), '.')))
+            path = "downstream"
+            if length(repo_and_subdir) > 1
+              length(repo_and_subdir) == 2 || error("Bad repo input.")
+              subdir = last(repo_and_subdir)
+              path = joinpath(path, subdir)
+              pkg_name = last(split(repo, '/'))
+            end
+            Pkg.develop(PackageSpec(path))
             # force it to use this PR's version of the package
             Pkg.develop(PackageSpec(path="."))  # resolver may fail with main deps
             Pkg.update()
-            pkg_name = string(first(split(last(split("${{ inputs.repo }}", '/')), '.')))
             Pkg.test(pkg_name)  # resolver may fail with test time deps
           catch err
             err isa Pkg.Resolve.ResolverError || rethrow()