Rename the attributecode module to aboutcode

[tdruez]

https://github.com/nexB/aboutcode-toolkit/tree/develop/src/attributecode

The main module name should match the library name.
This is confusing when using aboutcode-toolkit as a library.

[tdruez]

@pombredanne Is #313 properly merged?
The module name is still attributecode in current develop https://github.com/nexB/aboutcode-toolkit/tree/develop/src/attributecode

[pombredanne]

@tdruez #313 has not been merged and is deleted now. This is moved to v4