fix: PURLs with "nuget" type are dotnet packages (#2466)

Otherwise, Grype won't match on well-formed .NET purls from other SBOM
tools.

Signed-off-by: Will Murphy <will.murphy@anchore.com>

syft/pkg/language_test.go

@@ -38,6 +38,10 @@ func TestLanguageFromPURL(t *testing.T) {
 			purl: "pkg:dotnet/Microsoft.CodeAnalysis.Razor@2.2.0",
 			want: Dotnet,
 		},
+		{
+			purl: "pkg:nuget/Newtonsoft.Json@13.0.0",
+			want: Dotnet,
+		},
 		{
 			purl: "pkg:cargo/clap@2.33.0",
 			want: Rust,