Log module search paths when using -v

Even though these can be verbose, they are often important when diagnosing
issues related to import resolution.

Also tweak the format we use for logging these.

Example of what it looks like now:

```
LOG:  Module Search Paths:
LOG:      python_path:
LOG:          t
LOG:          /Users/jukka/src/mypy
LOG:      mypy_path:
LOG:          (empty)
LOG:      package_path:
LOG:          /Users/jukka/venv/mypy/lib/python3.7/site-packages
LOG:      typeshed_path:
LOG:          /Users/jukka/src/mypy/mypy/typeshed/stdlib/3.7
LOG:          /Users/jukka/src/mypy/mypy/typeshed/stdlib/3.6
LOG:          /Users/jukka/src/mypy/mypy/typeshed/stdlib/3
LOG:          /Users/jukka/src/mypy/mypy/typeshed/third_party/3
LOG:          /Users/jukka/src/mypy/mypy/typeshed/stdlib/2and3
LOG:          /Users/jukka/src/mypy/mypy/typeshed/third_party/2and3
LOG:          /usr/local/lib/mypy
```

mypy/build.py

diff --git a/mypy/build.py b/mypy/build.py
index f372f3d..a8172b6 100644
--- a/mypy/build.py
+++ b/mypy/build.py
@@ -2548,16 +2548,17 @@
     for conf_name, conf_value in configuration_vars:
         manager.log("{:24}{}".format(conf_name + ":", conf_value))
 
-    # Complete list of searched paths can get very long, put them under TRACE
+    manager.log("Module Search Paths:")
     for path_type, paths in manager.search_paths._asdict().items():
         if not paths:
-            manager.trace("No %s" % path_type)
+            manager.log("    %s:" % path_type)
+            manager.log("        (empty)")
             continue
 
-        manager.trace("%s:" % path_type)
+        manager.log("    %s:" % path_type)
 
         for pth in paths:
-            manager.trace("    %s" % pth)
+            manager.log("        %s" % pth)
 
 
 # The driver