Improve declared license collection from nuspec

[pombredanne]

... and accordingly the detection proper.
For instance in https://globalcdn.nuget.org/packages/specflow.mstest.3.1.89.nupkg this should be reported for SpecFlow.MsTest.nuspec and we get only this today: "declared_license": "https://aka.ms/deprecateLicenseUrl",

    <requireLicenseAcceptance>false</requireLicenseAcceptance>
    <license type="file">LICENSE.txt</license>
    <licenseUrl>https://aka.ms/deprecateLicenseUrl</licenseUrl>

Eventually this should be a candidate too for #1364 and #2616