Partially fixes #6068

Context

MSBuildFileSystemBase was introduced as a replacement of IFileSystem in #5546. It was added as an abstract class so potentially with the same constraints as an interface:

• Versioning constraints. We can't add a new public member to the class, unless we make it non-abstract and provide a default implementation of new members.
• Usability constraints. It is not possible to use MSBuildFileSystemBase to intercept calls or override a subset of them because the default MSBuild-provided file system is not exposed.

Changes Made

To address the constraints above, I am making the class non-abstract and having it by default forward all calls to the default MSBuild file system. This way the caller may choose to override only a subset of methods (e.g. file enumeration and existence/metadata checks) leaving the rest as is (e.g. actual file reads).

As a result I was able to delete both IFileSystemAdapter.cs and MSBuildFileSystemAdapter.cs because they're not needed anymore.

The difference between IFileSystem methods and public MSBuildFileSystemBase methods was just one method having a different name. I have renamed the one on the internal interface, assuming that this does not cause any troubles with the old NuGet, as tested by TestOldNuget(). This enabled further simplification and reduce the number of the "adapter" call hops.

With the changes in this PR, we have:

• An internal interface IFileSystem, kept around for compat reasons.
• A public class MSBuildFileSystemBase, which implements IFileSystem with virtual methods. The class is meant to be derived from and its methods overridden.

If we find ourselves adding new methods to the contract, it won't break existing MSBuildFileSystemBase because the new calls will just call through to the default implementation. If the change we need to make requires an orchestrated behavior change, we can always add a virtual 'version' prop, overriding which the derived class tells us which version of the contract it supports.

Testing

Existing unit test coverage.

Notes

This change is meant to be non-breaking. I'm basically just making an abstract class non-abstract (would have to be done at some point anyways if we were to add a new method). The rest of the changes are internal.