Split it up. (#366)

Summary:
Pull Request resolved: #366

Split up testslide into 3 separate libraries
- core to house the core functionality around mocking
- dsl contains the testslide dsl
- executor has the runner and import profiler

Differential Revision: D55324794

tests/cli_unittest.py

@@ -39,7 +39,7 @@ def run_testslide(
         args = [
             sys.executable,
             "-m",
-            "testslide.cli",
+            "testslide.executor.cli",
         ]
         if show_testslide_stack_trace:
             args.append("--show-testslide-stack-trace")