wipe back-end build artifacts before creating new ones

[alexcjohnson]

Not quite sure if this issue should be resolved in the component generator or in the component boilerplate, but when we're generating new Python/R/Julia files we should find a way to wipe the old ones first. Otherwise certain changes can leave obsolete files hanging around:

• If you change the R/Julia prefixes
• If you change the name of a component
• If you delete a component.

came up in https://github.com/plotly/dash-vtk/pull/6/files#r565645444

[emilykl]

@alexcjohnson Totally agree, this is needed!

Also related: There's some weird behaviour with the NAMESPACE file (which I think is part of the R components generation process?) containing lines for components that no longer exist.