Target .NET Framework 4.5

It turns out .NET Standard 2.0 is cool in practice, but bloats apps with shim files. This library is fairly minimal at the moment, so I'm adding a .NET Framework explicit TFM support to keep the library minimal

Author: natemcmaster

src/CommandLineUtils/McMaster.Extensions.CommandLineUtils.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFramework>netstandard2.0</TargetFramework>
+    <TargetFrameworks>net45;netstandard2.0</TargetFrameworks>
     <GenerateDocumentationFile>true</GenerateDocumentationFile>
     <IsPackable>true</IsPackable>
     <Description>Command-line parsing API. A community-maintained fork of Microsoft.Extensions.CommandLineUtils, plus extras.

src/CommandLineUtils/Utilities/DotNetExe.cs

@@ -36,6 +36,9 @@ public static string FullPathOrDefault()
 
         private static string TryFindDotNetExePath()
         {
+#if NET45
+            return "dotnet.exe";
+#elif NETSTANDARD2_0
             var fileName = FileName;
 
             if (RuntimeInformation.IsOSPlatform(OSPlatform.Windows))
@@ -75,6 +78,7 @@ private static string TryFindDotNetExePath()
             return File.Exists(muxer)
                 ? muxer
                 : null;
+#endif
         }
     }
 }