Add method to discover artefact paths

Add apathto method to discover artefact paths.

Add dictionary comprehension to pair build target names and artefact
paths to each version object. This will ensure that the build artefacts
for the current build is accessible via the current_version key in
html_context.

Add modifier to replace slashes in branch names with hyphens for file
naming purposes. This will ensure that the file name is not split and a
truncated version used for the artefact name.

Add shutil.py as a third party library to ensure backwards compatability
can be achieved with python 3.6. shutil.copytree only accepts the
dir_exists_ok argument from python 3.8.

Add unit tests and update metadata schema with appropriate mocked
objects for app configuration.

Author: samuel-emrys

setup.py

@@ -23,7 +23,7 @@
     version="0.2.4",
     install_requires=["sphinx >= 2.1"],
     license="BSD",
-    packages=["sphinx_multiversion"],
+    packages=["sphinx_multiversion", "sphinx_multiversion.lib"],
     entry_points={
         "console_scripts": [
             "sphinx-multiversion=sphinx_multiversion:main",