title: why is scons complaining "implicit dependency foo not found, needed by target bar" when foo exists?
body: <p>Scons is complaining </p>
  
  <pre><code>  implicit dependency foo not found, needed by target bar
  </code></pre>
  
  <p>foo is created in my SCons build flow, and then consumed in the creation of target bar.  After foo is created, SCons complains that it doesn't exist when it re-checks the dependencies before running bar.  Why is SCons caching the existence check for foo?  </p>
  
  <p>After I get the failure, if I remove .sconsign.dblite and rerun, SCons finds the file and keeps running.</p>