Migrate old graph to legacy directory/namespace

[ChuckHastings]

Code cleanup. We want to migrate the new graph object into the cugraph namespace. Moving the legacy graph objects into a legacy namespace to clear the way.

[codecov-commenter]

Codecov Report

Merging #1675 (164cfc7) into branch-21.08 (1030a49) will decrease coverage by 0.11%.
The diff coverage is n/a.

@@               Coverage Diff                @@
##           branch-21.08    #1675      +/-   ##
================================================
- Coverage         59.77%   59.66%   -0.12%     
================================================
  Files                80       80              
  Lines              3540     3540              
================================================
- Hits               2116     2112       -4     
- Misses             1424     1428       +4     

Impacted Files	Coverage Δ
python/cugraph/structure/number_map.py	63.92% <0.00%> (-1.18%)	⬇️
python/cugraph/utilities/utils.py	72.34% <0.00%> (-0.71%)	⬇️

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[BradReesWork]

@gpucibot merge