Generate hashes in requirements.txt

[sbidoul]

This will need to be implemented in pip freeze first.

[sbidoul]

Pip feature in progress: pypa/pip#10771

[sbidoul]

pip install --report and pip inspect are now available. Question is if we do a version of pip-deepfreeze that requires that minimum pip version or if we support both.

[sbidoul]

To implement this we need pip to support hash checking in presence of

• Direct URLs to immutable commits (in progress at Validate VCS urls in hash-checking mode using their commit hashes pypa/pip#11968)
• editable installs of local directories (which should be reasonable as, arguably, users trust their local file system): editable cannot be installed when requiring hashes pypa/pip#4995