Rename top-level package to ‘zeronet’

To allow importing and distributing the ZeroNet code base, the top-level Python package should be the name which gets imported.

An obvious name would be `zeronet`. This can be done by renaming the `src` directory to `zeronet`, and changing all the existing references from the old name to the new.

Start by reviewing the top-level src directory and inventorying references to its current package name across the repository. Rename src to zeronet and update the existing references so the code can be imported and distributed under the zeronet package name.

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