Minimal example of DUB dependency clashes

This is a simple example of the dependency resolution failure described
in dlang/dub#1732.  Running `dub build` fails
with the error:

    Unresolvable dependencies to package taggedalgebraic:
      eventcore 0.8.43 depends on taggedalgebraic >=0.10.12 <0.12.0-0
      std_data_json 0.18.3 depends on taggedalgebraic ~>0.10.1

The reported failure in fact has an easily recognizable resolution, but
for some reason DUB is unable to find it.  It is not 100% clear if this
is the real dependency clash or not, but it is interesting to note that
if `dep2` (which has no dependencies of its own) is removed from the
list of dependencies of the main project, the resolution failure also
disappears.

dep1/dub.json

@@ -0,0 +1,10 @@
+{
+    "name": "dep1",
+
+    "targetType": "sourceLibrary",
+
+    "dependencies": {
+        "std-experimental-xml": ">=0.1.7 <0.2.0",
+        "unit-threaded:assertions": ">=0.8.0 <0.10.0"
+    }
+}

dep1/src/dep1.d

@@ -0,0 +1,6 @@
+module dep1;
+
+string foo ()
+{
+    return "foo";
+}

dep2/dub.json

@@ -0,0 +1,4 @@
+{
+    "name": "dep2",
+    "targetType": "sourceLibrary"
+}

dep2/src/dep1.d

@@ -0,0 +1,6 @@
+module dep1;
+
+string foo ()
+{
+    return "foo";
+}

dub.json

@@ -0,0 +1,19 @@
+{
+    "name": "vibe-deps-failure",
+    "authors": ["Joseph Wakeling"],
+    "description": "Simple example of DUB dependency resolution failure",
+
+    "dependencies": {
+        "dep1": { "path": "dep1" },
+        "dep2": { "path": "dep2" },
+        "std_data_json": ">=0.18.3 <0.19.0",
+        "pyd": ">=0.10.5 <0.11.0",
+        "vibe-core": ">=1.6.0 <2.0.0",
+        "vibe-d:redis": ">=0.8.4 <0.9.0",
+        "vibe-d:tls": ">=0.8.4 <0.9.0"
+    },
+    "subConfigurations": {
+        "pyd": "python36",
+        "vibe-d:tls": "openssl-1.1"
+    }
+}

src/app.d

@@ -0,0 +1,6 @@
+void main ()
+{
+    import dep1 : foo;
+    import std.stdio : writefln;
+    writefln!"Hello, I'm just a stub for %s"(foo());
+}