EbuildBuild: skip the fetch queue when possible

EbuildBuild: skip the fetch queue when possible

Since commit f07f8386e945b48358c11c121960e4833c539752, it was possible
for EbuildBuild to wait on the fetch queue even in cases in which all
required files had been previously fetched. Now this case is optimized
to skip the fetch queue, as discribed in bug #375331, comment #2.