add PaketExeImage property

- `assembly` paket is .net app, require mono on unix/osx
- `native`. paket is a native binary

Author: enricosada

src/Paket.Core/embedded/Paket.Restore.targets

@@ -13,21 +13,24 @@
     <PaketLockFilePath>$(PaketRootPath)paket.lock</PaketLockFilePath>
     <PaketBootstrapperStyle>classic</PaketBootstrapperStyle>
     <PaketBootstrapperStyle Condition="Exists('$(PaketToolsPath)paket.bootstrapper.proj')">proj</PaketBootstrapperStyle>
+    <PaketExeImage>assembly</PaketExeImage>
+    <PaketExeImage Condition=" '$(PaketBootstrapperStyle)' == 'proj' ">native</PaketExeImage>
     <MonoPath Condition="'$(MonoPath)' == '' And Exists('/Library/Frameworks/Mono.framework/Commands/mono')">/Library/Frameworks/Mono.framework/Commands/mono</MonoPath>
     <MonoPath Condition="'$(MonoPath)' == ''">mono</MonoPath>
     <!-- Paket command -->
     <PaketExePath Condition=" '$(PaketExePath)' == '' AND Exists('$(PaketToolsPath)paket')">$(PaketToolsPath)paket</PaketExePath>
     <PaketExePath Condition=" '$(PaketExePath)' == '' AND Exists('$(PaketRootPath)paket.exe')">$(PaketRootPath)paket.exe</PaketExePath>
-    <PaketExePath Condition=" '$(PaketExePath)' == '' ">$(PaketToolsPath)paket.exe</PaketExePath>
-    <PaketCommand Condition=" '$(OS)' == 'Windows_NT'">"$(PaketExePath)"</PaketCommand>
-    <PaketCommand Condition=" '$(OS)' != 'Windows_NT' ">$(MonoPath) --runtime=v4.0.30319 "$(PaketExePath)"</PaketCommand>
 
-    <!-- .net core fdd -->
-    <_PaketExeExtension>$([System.IO.Path]::GetExtension("$(PaketExePath)"))</_PaketExeExtension>
-    <PaketCommand Condition=" '$(_PaketExeExtension)' == '.dll' ">dotnet "$(PaketExePath)"</PaketCommand>
+    <PaketExePath Condition=" '$(PaketExePath)' == '' AND '$(OS)' == 'Windows_NT' ">$(PaketToolsPath)paket.exe</PaketExePath>
+    <PaketExePath Condition=" '$(PaketExePath)' == '' AND '$(OS)' != 'Windows_NT' AND '$(PaketExeImage)' == 'assembly' ">$(PaketToolsPath)paket.exe</PaketExePath>
+    <PaketExePath Condition=" '$(PaketExePath)' == '' AND '$(OS)' != 'Windows_NT' AND '$(PaketExeImage)' == 'native' ">$(PaketToolsPath)paket</PaketExePath>
 
-    <!-- no extension is a shell script -->
-    <PaketCommand Condition=" '$(_PaketExeExtension)' == '' ">"$(PaketExePath)"</PaketCommand>
+    <!-- Paket command -->
+    <_PaketExeExtension>$([System.IO.Path]::GetExtension("$(PaketExePath)"))</_PaketExeExtension>
+    <PaketCommand Condition=" '$(PaketCommand)' == '' AND '$(_PaketExeExtension)' == '.dll' ">dotnet "$(PaketExePath)"</PaketCommand>
+    <PaketCommand Condition=" '$(PaketCommand)' == '' AND '$(OS)' == 'Windows_NT'">"$(PaketExePath)"</PaketCommand>
+    <PaketCommand Condition=" '$(PaketCommand)' == '' AND '$(OS)' != 'Windows_NT' AND '$(_PaketExeExtension)' == '.exe' ">$(MonoPath) --runtime=v4.0.30319 "$(PaketExePath)"</PaketCommand>
+    <PaketCommand Condition=" '$(PaketCommand)' == '' AND '$(OS)' != 'Windows_NT'">"$(PaketExePath)"</PaketCommand>
 
     <PaketBootStrapperExePath Condition=" '$(PaketBootStrapperExePath)' == '' AND Exists('$(PaketRootPath)paket.bootstrapper.exe')">$(PaketRootPath)paket.bootstrapper.exe</PaketBootStrapperExePath>
     <PaketBootStrapperExePath Condition=" '$(PaketBootStrapperExePath)' == '' ">$(PaketToolsPath)paket.bootstrapper.exe</PaketBootStrapperExePath>