Move main source code into src subdirectory

This avoids certain problems related to the entire repo root appearing
on the import path, usually when running Python/pip from the repo root
or when using an editable install.

setup.cfg

@@ -43,15 +43,15 @@ keywords =
 zip_safe = False
 python_requires = >=3.6
 packages = find:
+package_dir =
+	= src
 
 [options.package_data]
 rsrcfork =
 	py.typed
 
 [options.packages.find]
-include =
-	rsrcfork
-	rsrcfork.*
+where = src
 
 [options.entry_points]
 console_scripts =
@@ -77,7 +77,7 @@ blank-lines-indent = always
 indent-tabs-def = 1
 
 [mypy]
-files=rsrcfork/**/*.py
+files=src/**/*.py
 python_version = 3.6
 
 disallow_untyped_calls = True