Refactor DotNetAliases: Extract methods into a separate partial class

From @Marusyk in [discussion](https://github.com/cake-build/cake/pull/4392#discussion_r1838854200):

>  ... Consider using partial classes for this DotNetAliases to better grouping and navigation
e.g.
- DotNetAliases.cs
.... - DotNetAliases.Sln.cs
.... - DotNetAliases.Package.cs