Create new PropertyGraph source dir under cugraph/python

[rlratzel]

Who: end users, cugraph developers
What: move PropertyGraph to a separate subdir in the source tree
Why: This gives PG a place for new development with minimal disruption to existing (albeit experimental) PG. This is an incremental step towards a separate conda/pip package to allow for PG-specific dependencies, to be done in a separate release.

• property-graph subdir under cugraph/python (PG code stays in cugraph monorepo) - or do we want a different name?
• Should not depend on cugraph (but can depend on PLC or other lower-level libs if necessary?).

Tasks

Beta Give feedback

1. Pick a name (should be consistent with other the naming conventions of other source dirs that already have conda/pip packages wrt use of hyphens, etc.)
2. Update CI scripts to build PG and run tests from new location

[rlratzel]

• we'll be creating a completely new implementation with no guarantees of backwards-compatibility with cugraph.experimental.property_graph
  • we'll deprecate experimental PG starting in 23.08
  • cugraph-pg will provide a "comparable" feature set to experimental
    • add heterogeneous data, query, extract subgraph

[rlratzel]

rapidsai/cugraph#3618