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).