Proj5006: Included files should exist

Files included in an SLNX solution should exist on disk. Referencing files that do not exist or have been moved or deleted can cause confusion and clutter in the solution explorer.

Non-Compliant

<Solution>
  <Folder Name="/Solution Items/">
    <File Path="non-existing.file" />
    <File Path="existing.file" />
  </Folder>
  <Project Path="src/SomeProject.csproj" />
</Solution>

Compliant

<Solution>
  <Folder Name="/Solution Items/">
    <File Path="existing.file" />
  </Folder>
  <Project Path="src/SomeProject.csproj" />
</Solution>