refactor!: use Testably.Abstractions.FileSystem.Interface (#1196)

In order to simplify extensibility, move the filesystem interfaces to a separate package ["Testably.Abstractions.FileSystem.Interface"](https://www.nuget.org/packages/Testably.Abstractions.FileSystem.Interface), that will keep its version in sync with the .NET versioning scheme (starting with v9.0.0).

Author: vbreuss

Directory.Packages.props

@@ -7,6 +7,7 @@
     <PackageVersion Include="Microsoft.NETFramework.ReferenceAssemblies.net462" Version="1.0.3" />
     <PackageVersion Include="IsExternalInit" Version="1.0.3" />
     <PackageVersion Include="Nullable" Version="1.3.1" />
+    <PackageVersion Include="Testably.Abstractions.FileSystem.Interface" Version="9.0.0" />
   </ItemGroup>
   <ItemGroup>
     <PackageVersion Include="BenchmarkDotNet" Version="0.14.0" />

System.IO.Abstractions.sln

@@ -90,6 +90,7 @@ Global
 		{919888D2-E37D-40E7-8AD0-600F9429316D} = {BCEC61BD-4941-41EC-975A-ACEFC7AC1780}
 		{2BE9161B-A3F3-4511-81DB-DB1DCB6375C9} = {BBF7AD8D-5522-48C0-A906-00CBB72308A0}
 		{D905E09D-6DC3-4F7C-8E83-82FADAE2C9E5} = {2BE9161B-A3F3-4511-81DB-DB1DCB6375C9}
+		{1B8388D2-58A7-47B8-89EC-C5A94A0FEED5} = {C078E0B6-9747-475F-A999-B9E775DF6643}
 	EndGlobalSection
 	GlobalSection(ExtensibilityGlobals) = postSolution
 		SolutionGuid = {8885C59C-F6A0-4C2F-A3BC-B720E9BD161F}

src/TestableIO.System.IO.Abstractions.TestingHelpers/TestableIO.System.IO.Abstractions.TestingHelpers.csproj

@@ -7,7 +7,7 @@
         <PackageIcon>icon_256x256.png</PackageIcon>
     </PropertyGroup>
     <ItemGroup>
-        <ProjectReference Include="../TestableIO.System.IO.Abstractions/TestableIO.System.IO.Abstractions.csproj" />
+        <PackageReference Include="Testably.Abstractions.FileSystem.Interface" />
         <ProjectReference Include="../TestableIO.System.IO.Abstractions.Wrappers/TestableIO.System.IO.Abstractions.Wrappers.csproj" />
     </ItemGroup>
     <ItemGroup>

src/TestableIO.System.IO.Abstractions.Wrappers/TestableIO.System.IO.Abstractions.Wrappers.csproj

@@ -19,6 +19,6 @@
     <None Include="..\..\images\icon_256x256.png" Pack="true" PackagePath="\" />
   </ItemGroup>
   <ItemGroup>
-    <ProjectReference Include="..\TestableIO.System.IO.Abstractions\TestableIO.System.IO.Abstractions.csproj" />
+    <PackageReference Include="Testably.Abstractions.FileSystem.Interface" />
   </ItemGroup>
 </Project>