[Bug] Files without managed metadata break docfx

[kemocca]

Unmanaged dependencies break generation of documentation process with exception:

MetadataFileNotSupportedException: PE file does not contain any managed metadata.
  at ctor(MetadataFileKind kind, string fileName, PEReader reader, MetadataReaderOptions
     metadataOptions) in MetadataFile.cs:245
  at ctor(string fileName, PEReader reader, MetadataReaderOptions metadataOptions) in PEFile.cs:49
  at ctor(string fileName, PEStreamOptions streamOptions, MetadataReaderOptions metadataOptions) in
     PEFile.cs:39
  at IEnumerable<string> GetReferenceAssemblies(string assemblyPath, MetadataReference[] references)
     in CompilationHelper.cs:169
  at (Compilation, IAssemblySymbol) CreateCompilationFromAssembly(string assemblyPath, bool
     includePrivateMembers, params MetadataReference[] references) in CompilationHelper.cs:109
  at async (List<ValueTuple<IAssemblySymbol, Compilation>> symbol) Compile(ExtractMetadataConfig
     config) in DotnetApiCatalog.Compile.cs:113
  at async Task <Exec>g__Build|6_0(ExtractMetadataConfig config, DotnetApiOptions options) in
     DotnetApiCatalog.cs:85
  at async Task Exec(MetadataJsonConfig config, DotnetApiOptions options, string configDirectory,
     string outputDirectory) in DotnetApiCatalog.cs:71
  at void <Execute>b__0() in DefaultCommand.cs:45
  at int Run(LogOptions options, Action run) in CommandHelper.cs:48
  at int Execute(CommandContext context, Options options, CancellationToken cancellationToken) in
     DefaultCommand.cs:31
  at int Execute(CommandContext context, TSettings settings) in CancellableCommandBase.cs:24
  at Task<int> Execute(CommandContext context, CommandSettings settings) in CommandOfT.cs:40
  at async Task<int> Execute(CommandTree leaf, CommandTree tree, CommandContext context,
     ITypeResolver resolver, IConfiguration configuration) in CommandExecutor.cs:166

To Reproduce
Steps to reproduce the behavior:

1. Setup metadata section like:

"metadata": [
  {
    "src": [
      {
        "src": "../../src",
        "files": ["**/bin/Release/**.dll"]
      }
    ],
    "dest": "api"
  }
],

2. Place unmanaged dll into Release folder for simulating unmanaged dependency;
3. Run docfx docfx.json --serve

Expected behavior
Just skip unmanaged items.

Context:

• OS: Windows
• Docfx version: 2.78.3+850a61c2ecb83b0bda21448f1393e480934556fd