GitVersionTask - Removed additional MsBuild dependency.

- No longer dependent on Microsoft.Build.Tasks.Core.
- Slight refactor of static type initialisers to catch exception for inspection with debugger attach during task execution.

Author: dazinator

src/GitVersionTask.MsBuild/GitVersionTask.MsBuild.csproj

@@ -5,11 +5,11 @@
         <LangVersion>8.0</LangVersion>
         <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
     </PropertyGroup>
-
+   
     <ItemGroup>
-        <PackageReference Include="System.Runtime.Loader" Version="4.3.0" />
+        <PackageReference Condition="'$(TargetFramework)' == 'netstandard2.0'" Include="System.Runtime.Loader" Version="4.3.0" PrivateAssets="All" />
         <PackageReference Include="Microsoft.Build" Version="16.4.0" NoWarn="NU1701" />
-        <PackageReference Include="Microsoft.Build.Tasks.Core" Version="16.4.0" />
+        <PackageReference Include="Microsoft.Build.Framework" Version="16.4.0" />
     </ItemGroup>
 
     <ItemGroup>

src/GitVersionTask.MsBuild/GitVersionTaskBase.cs

@@ -1,10 +1,16 @@
+using GitVersionTask.MsBuild;
 using Microsoft.Build.Framework;
 using Microsoft.Build.Utilities;
 
 namespace GitVersion.MSBuildTask
 {
-    public abstract class GitVersionTaskBase : Task
+    public abstract class GitVersionTaskBase : ITask
     {
+        protected GitVersionTaskBase()
+        {
+            this.Log = new TaskLoggingHelper(this);
+        }
+
         [Required]
         public string SolutionDirectory { get; set; }
 
@@ -13,5 +19,22 @@ public abstract class GitVersionTaskBase : Task
         public bool NoFetch { get; set; }
 
         public bool NoNormalize { get; set; }
+        public IBuildEngine BuildEngine { get; set; }
+        public ITaskHost HostObject { get; set; }
+
+        public TaskLoggingHelper Log { get; }
+
+        public bool Execute()
+        {
+            if (TaskProxy.InitialiseException != null)
+            {
+                throw TaskProxy.InitialiseException;
+            }
+
+            return OnExecute();
+        }
+
+        protected abstract bool OnExecute();
+
     }
 }