Allow building deprecated modules without deprecation warnings

The compiler issues a deprecation warning only on a
"non-deprecated" -> "deprecated" transition.  As such, building or
testing Dub packages which have deprecated modules when a
dub_test_root.d file needs to be generated will cause deprecation
warnings.

This in turn makes it impossible to use `buildRequirements
"disallowDeprecations"` in this situation.

Fix this by simply making the auto-generated root
module (dub_test_root) deprecated. As it is the compiler's entry
point, the module itself will not cause a deprecation warning, and it
importing deprecating modules will not cause deprecation warnings (as
there is no longer a transition of deprecation state).

Author: CyberShadow

source/dub/project.d

@@ -1970,6 +1970,7 @@ public class SelectedVersions {
 
 /// The template code from which the test runner is generated
 private immutable TestRunnerTemplate = q{
+deprecated // allow silently using deprecated symbols
 module dub_test_root;
 
 import std.typetuple;

test/issue2650-deprecated-modules/.no_build

@@ -0,0 +1,3 @@
+name "issue2650"
+targetType "sourceLibrary"
+buildRequirements "disallowDeprecations"

test/issue2650-deprecated-modules/.no_run

@@ -0,0 +1 @@
+deprecated module test;