Move sources in `src` directory

…as recommended by Python packaging.

• Move silkaj and g1_monetary_license directories in src directory
• Add this commit in a .git-blame-ignore-revs?

Issue

Can’t find a working way for both the wheel and dev usage to have it working.

• with silkaj and g1_monetary_license in src directory, pyproject.toml has to be adapted to include src/g1_monetary_license, but then when installing the wheel, its installed into src and therefore not importable.
• with only silkaj in src and g1_monetary_license at the root of the repository, the wheel is working, but dev env does no longer manage to import g1_monetary_license.