FindSherpa.cmake Improvement, master branch (2020.06.03.)

This is a sibling of !683 (merged). It achieves the same effect, just using a built-in feature of the lcg_external_module(...) function. (One which is not present in the 1.0 branch.)

It also sets up the ${SHERPA_Sherpa_EXECUTABLE} and ${SHERPA_Sherpa-config_EXECUTABLE} cmake variables, which may prove useful in the future.

Cc. @cgutscho

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merged

mentioned in commit dc553b2e