Add missing local metadata and allow local only metadata

[emaheuxPEREN]

Purpose of This Pull Request

Please check the relevant option by placing an "X" inside the brackets:

• Documentation update
• Bug fix (specs conformance)
• New feature
• Other (please explain):

Overview of Changes

Two-fold:

• fix/specs: we add missing metadata keys in LocalPackageInfo, especially License-Expression
• feat: we add the ability for the user to retrieve package metadata only from local by setting an empty --pypi-api argument

Related Issue

No opened issue but when dealing with a non-public PyPI package only having License-Expression metadata, licensecheck was failing to retrieve its license from local metadata.