Easy-mode production of executable from sdist

It would be nice to have a command like pyoxidizer --build-executable-easy-mode my_module_sdist.tar.gz that "does the right thing" and produces a single-file executable from the given .tar.gz. Taking this representative sdist as an example, the sdist contains a representative/__main__.py, so when installed in a Python environment it would be runnable as python -m representative, so it makes sense to just build an executable named representative for it. There are lot of rich features available when writing a pyoxidizer.bzl, but... let me offer the hypotheses that learning to write a pyoxidizer.bzl oughtn't strictly be necessary to the simplest use cases and that there's also a lot of value to be found in just transforming sdists to executables.

Start with the pyoxidizer --build-executable-easy-mode command and the representative sdist containing representative/__main__.py; compare this proposed flow with the existing pyoxidizer.bzl workflow. Done means the supplied .tar.gz can be transformed into a single-file executable named representative without requiring a pyoxidizer.bzl.