Include cpack to generate source packages

[ajschmidt8]

This PR adds include(CPack) to CMakeLists.txt to support generating source packages for cugraph.

cc: @trxcllnt

[ajschmidt8]

rerun tests

[codecov-commenter]

Codecov Report

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@@               Coverage Diff               @@
##             branch-21.08    #1760   +/-   ##
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  Coverage                ?   59.75%           
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  Files                   ?       77           
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  Hits                    ?     2105           
  Misses                  ?     1418           
  Partials                ?        0           

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