Initial version of pylibcugraph source tree and build script updates

[rlratzel]

Initial version of pylibcugraph source tree and build script updates plus connected_components placeholder.

Part of #1782

Tested by running the new tests part of this PR. Also manually tested by building pylibcugraph importing it interactively, and confirming pylibcugraph.weakly_connected_components raise NotImplementedError

Note: there is likely still some cleanup to do in the various setup files, since those were copied as-is from cugraph and modified only to work with pylibcugraph dir names.

[rlratzel]

rerun tests

[codecov-commenter]

Codecov Report

Merging #1787 (9f14c60) into branch-21.10 (bf64c2c) will increase coverage by 11.67%.
The diff coverage is n/a.

❗ Current head 9f14c60 differs from pull request most recent head fec9351. Consider uploading reports for the commit fec9351 to get more accurate results

@@                Coverage Diff                @@
##           branch-21.10    #1787       +/-   ##
=================================================
+ Coverage         59.85%   71.52%   +11.67%     
=================================================
  Files                77      134       +57     
  Lines              3547     8126     +4579     
=================================================
+ Hits               2123     5812     +3689     
- Misses             1424     2314      +890     

Impacted Files	Coverage Δ
python/cugraph/cugraph/__init__.py	100.00% <ø> (ø)
python/cugraph/cugraph/_version.py	44.40% <ø> (ø)
python/cugraph/cugraph/centrality/__init__.py	100.00% <ø> (ø)
...graph/cugraph/centrality/betweenness_centrality.py	90.00% <ø> (ø)
...thon/cugraph/cugraph/centrality/katz_centrality.py	89.47% <ø> (ø)
python/cugraph/cugraph/comms/comms.py	34.52% <ø> (ø)
python/cugraph/cugraph/community/__init__.py	70.37% <ø> (ø)
python/cugraph/cugraph/community/ecg.py	100.00% <ø> (ø)
python/cugraph/cugraph/community/egonet.py	92.30% <ø> (ø)
...ython/cugraph/cugraph/community/ktruss_subgraph.py	84.37% <ø> (ø)
... and 177 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update fc0aef7...fec9351. Read the comment docs.

[jjacobelli]

rerun tests

[rlratzel]

rerun tests

[BradReesWork]

@gpucibot merge