hghave: make bzr checks stricter

My bzr does not have bzrlib.revisionspec.RevisionSpec,
and thus tests were failing because convert refused to believe in bzr,
but hghave without this change thought it was available.

tests/hghave.py

@@ -110,6 +110,10 b' def has_baz():'
 def has_bzr():
     try:
         import bzrlib
+        import bzrlib.bzrdir
+        import bzrlib.errors
+        import bzrlib.revision
+        import bzrlib.revisionspec.RevisionSpec
         return bzrlib.__doc__ is not None
     except ImportError:
         return False