Manifest.ReadFrom fails on any nuspec with LicenseExpression

[ericstj]

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Details about Problem

NuGet product used (NuGet.exe | VS UI | Package Manager Console | dotnet.exe):
dotnet.exe

NuGet version (x.x.x.xxx):
4.9.2.0

dotnet.exe --version (if appropriate):
2.2.103

Worked before? If so, with which NuGet version:
nope

Detailed repro steps so we can see the same problem

1. Generate a nuspec with LicenseExpression and save using Manifest.Save

2. Load that Nuspec with Manifest.ReadFrom

Expect: success in round-tripping nuspec

Actual:

System.Exception: The licenseUrl and license elements cannot be used together.
at NuGet.Packaging.Manifest.Validate(Manifest manifest) 
at NuGet.Packaging.Manifest.ReadFrom(Stream stream, Func`2 propertyProvider, Boolean validateSchema)

The same can be observed if you try to use Manifest.ReadFrom with any nuspec that's in a dotnet-pack'ed package that used LicenseExpression.

The culprit appears to be a bad equality check here:
https://github.com/NuGet/NuGet.Client/blob/91d2331328d55ab9f450af1fbf184fc1f261ed25/src/NuGet.Core/NuGet.Packaging/PackageCreation/Authoring/ManifestMetadata.cs#L336-L339

It's calling String.Equals and passing a URI.

dotnet/arcade#2091 (comment) /cc @tmat @nkolev92