This PR addresses #407 by fixing the regular expression used to determine whether a package reference is pinned or not.

Steps to Test

Before this PR, using the latest version (0.52.0), the following:

PS> dotnet script eval '#r \"nuget: Microsoft.Data.SqlClient,2.0.0-preview3.20122.2\"'

would issue the warning:

warn: Dotnet.Script.DependencyModel.Context.CachedRestorer[0]
      Unable to cache C:\Users\johndoe\AppData\Local\Temp\dotnet-script\A\Dotnet.Script\Eval\interactive\interactive\netcoreapp3.1\script.csproj. For caching and optimal performance, ensure that the script(s) references Nuget packages with a pinned version.

After checking out this PR's branch, the following:

PS> dotnet run -p .\src\Dotnet.Script -f netcoreapp3.1 -- eval '#r \"nuget: Microsoft.Data.SqlClient,2.0.0-preview3.20122.2\"'

will issue no warning about non-cache-ability of a floating (or an unpinned) reference.

Additional Notes

This PR also refactored parts to share the regular expressions for package references more consistently between ScriptParser and NuGetSourceReferenceResolver. Since these two reside in separate projects, I avoided a dependency between the two by rendering ScriptParser partial, moving shared parts to another partial definition of the same class in a separate file called ScriptParserInternals.cs, and finally linking that into the Dotnet.Script.DependencyModel.NuGet project. The partial definition in ScriptParserInternals.cs has no public modifier so ScriptParser will not get introduced into the public API of Dotnet.Script.DependencyModel.NuGet.