Add support for NuGet license files

[mus65]

fixes #1001 .

This adds support for license files that are defined in the NuGet package.

• if GitHub license resolution is disabled, it will prefer the license file to the license url (which has been deprecated for years) .
• if GitHub license resolution is enabled, it will only fall back to the license file if nothing was found with GitHub, so this shouldn't break anything.

Some NuGet Packages that this would help with are Lucene.Net, LibGit2Sharp and Oracle.ManagedDataAccess.Core .

[Falco20019]

Would love to see this in the next release.

[gitguardian]

️✅ There are no secrets present in this pull request anymore.

If these secrets were true positive and are still valid, we highly recommend you to revoke them.
While these secrets were previously flagged, we no longer have a reference to the
specific commits where they were detected. Once a secret has been leaked into a git
repository, you should consider it compromised, even if it was deleted immediately.
Find here more information about risks.

---

^{_{🦉 GitGuardian detects secrets in your source code to help developers and security teams secure the modern development process. You are seeing this because you or someone else with access to this repository has authorized GitGuardian to scan your pull request.}}

[mtsfoni]

Needs some small fixes, and I'll discuss if this should be the default behavior with a member of the core group.

[mtsfoni]

@mus65 are you still working on this. I would take over as soon as I got the feedback from the core group

[mus65]

@mtsfoni I'm done, just did a rebase because of a merge conflict (I merged master at first, but it triggered secret detection for some reason).