Clean up ORAS code

[isc-shuliu]

There is some code in ORAS that can be further improved for clarity.

• Method naming: I think Python methods don't have to use the "Py" suffix. They are first class methods just like ObjectScript methods. Thoughts?
• Argument naming. Some arguments are misleading, such as package are sometimes actually a concatenation of package:tag. Also, tag and version are no longer synonyms after V0.10.x fix translate semver tags #700. We need to be explicit about that.
• Inconsistent return type. Some python methods can return either a tuple or a comma-separated string based on an input flag. This is convenient for now but harder to maintain in the future.
• Redirection of stdout/stderr can wrapped with context managers for readability
• Misc changes. Some code be made more pythonic