[nativeaot] fix Exists() checks (#112995)

Context: dotnet/android#9846

Trying to build our `Mono.Android-Tests` suite for NativeAOT, it fails
with:

    EXEC Failed to load assembly 'System.IO'

Which is caused by passing:

    --root:System.IO

I couldn't find anything in the Android workload that would cause this.

It appears the `Microsoft.NETCore.Native.targets` have an issue caused
by this specific project:

* When adding to `@(_IlcRootedAssemblies)` it does an `Exists()` check

* So `Exists('System.IO.Compression')` is called. This is *true*
  because there is a folder in the working directory named
  `System.IO.Compression`!

https://github.com/dotnet/android/tree/main/tests/Mono.Android-Tests/Mono.Android-Tests/System.IO.Compression

* `%(FileName)` is then `System.IO` as `.Compression` is stripped as
  the `%(Extension)` metadata.

I believe the fix here is to use `System.IO.File.Exists()` instead of
MSBuild's `Exists()` as it returns `true` for files *and* folders.

Author: jonathanpeppers

src/coreclr/nativeaot/BuildIntegration/Microsoft.NETCore.Native.targets

@@ -218,16 +218,17 @@ The .NET Foundation licenses this file to you under the MIT license.
 
       <!-- Now process the raw ItemGroup into the form that ILC accepts: assembly names only -->
       <!-- Use the logic that ILLinker uses: if the file exists, this is a file name. Otherwise it's an assembly name -->
-      <_IlcRootedAssemblies Include="@(_IlcRootedAssembliesRaw->'%(Filename)')" Condition="Exists('%(Identity)')" />
-      <_IlcRootedAssemblies Include="@(_IlcRootedAssembliesRaw)" Condition="!Exists('%(Identity)')" />
-      <_IlcConditionallyRootedAssemblies Include="@(_IlcConditionallyRootedAssembliesRaw->'%(Filename)')" Condition="Exists('%(Identity)')" />
-      <_IlcConditionallyRootedAssemblies Include="@(_IlcConditionallyRootedAssembliesRaw)" Condition="!Exists('%(Identity)')" />
-      <_IlcTrimmedAssemblies Include="@(_IlcTrimmedAssembliesRaw->'%(Filename)')" Condition="Exists('%(Identity)')" />
-      <_IlcTrimmedAssemblies Include="@(_IlcTrimmedAssembliesRaw)" Condition="!Exists('%(Identity)')" />
-      <_IlcSingleWarnAssemblies Include="@(_IlcSingleWarnAssembliesRaw->'%(Filename)')" Condition="Exists('%(Identity)')" />
-      <_IlcSingleWarnAssemblies Include="@(_IlcSingleWarnAssembliesRaw)" Condition="!Exists('%(Identity)')" />
-      <_IlcNoSingleWarnAssemblies Include="@(_IlcNoSingleWarnAssembliesRaw->'%(Filename)')" Condition="Exists('%(Identity)')" />
-      <_IlcNoSingleWarnAssemblies Include="@(_IlcNoSingleWarnAssembliesRaw)" Condition="!Exists('%(Identity)')" />
+      <!-- Use System.IO.File.Exists() to avoid directories-->
+      <_IlcRootedAssemblies Include="@(_IlcRootedAssembliesRaw->'%(Filename)')" Condition="$([System.IO.File]::Exists('%(Identity)'))" />
+      <_IlcRootedAssemblies Include="@(_IlcRootedAssembliesRaw)" Condition="!$([System.IO.File]::Exists('%(Identity)'))" />
+      <_IlcConditionallyRootedAssemblies Include="@(_IlcConditionallyRootedAssembliesRaw->'%(Filename)')" Condition="$([System.IO.File]::Exists('%(Identity)'))" />
+      <_IlcConditionallyRootedAssemblies Include="@(_IlcConditionallyRootedAssembliesRaw)" Condition="!$([System.IO.File]::Exists('%(Identity)'))" />
+      <_IlcTrimmedAssemblies Include="@(_IlcTrimmedAssembliesRaw->'%(Filename)')" Condition="$([System.IO.File]::Exists('%(Identity)'))" />
+      <_IlcTrimmedAssemblies Include="@(_IlcTrimmedAssembliesRaw)" Condition="!$([System.IO.File]::Exists('%(Identity)'))" />
+      <_IlcSingleWarnAssemblies Include="@(_IlcSingleWarnAssembliesRaw->'%(Filename)')" Condition="$([System.IO.File]::Exists('%(Identity)'))" />
+      <_IlcSingleWarnAssemblies Include="@(_IlcSingleWarnAssembliesRaw)" Condition="!$([System.IO.File]::Exists('%(Identity)'))" />
+      <_IlcNoSingleWarnAssemblies Include="@(_IlcNoSingleWarnAssembliesRaw->'%(Filename)')" Condition="$([System.IO.File]::Exists('%(Identity)'))" />
+      <_IlcNoSingleWarnAssemblies Include="@(_IlcNoSingleWarnAssembliesRaw)" Condition="!$([System.IO.File]::Exists('%(Identity)'))" />
     </ItemGroup>
 
     <ItemGroup>