tp: several changes for Python API

1. Move everything into a new trace_processor folder inside the Python
folder and add an empty setup.py. Doing this will allow us to correctly
implement setup.py based standalone installation using pip down the line.
2. Add an __init__.py inside the new trace_processor folder to make it
a module we can use.
3. Rename trace_processor_http.py -> http.py now that we have the
wrapping trace_processor folder.
4. Rename api_main.py -> example.py to better reflect its intended
usecase.
5. Check in the trace_processor.proto descriptor (including modifying
the gen_binary_descriptor script to generate this).
6. Add a proto Python module which reads the descriptor and generates
proto factory functions.

These changes were verified to work with both bazel run and execing the
example.py script directly.

Change-Id: I7603edee8741632c6f8d1c7b8ea9d9d17a4c78cd
diff --git a/BUILD.extras b/BUILD.extras
index a8f3a45..cc5a335 100644
--- a/BUILD.extras
+++ b/BUILD.extras
@@ -70,16 +70,14 @@
 )
 
 perfetto_py_binary(
-    name = "api_main",
-    srcs = ["src/trace_processor/python/api_main.py"],
-    deps = [":api_main_lib"],
-    main = "src/trace_processor/python/api_main.py",
+    name = "trace_processor_py_example",
+    srcs = ["src/trace_processor/python/example.py"],
+    deps = [":trace_processor_py"],
+    main = "src/trace_processor/python/example.py",
     python_version = "PY3",
 )
 
 perfetto_py_library(
-    name = "api_main_lib",
-    srcs = [
-        "src/trace_processor/python/trace_processor_http.py",
-    ],
+    name = "trace_processor_py",
+    srcs = glob(['src/trace_processor/python/trace_processor/*.py']),
 )