Better support for complex wheel builds

[jvolkman]

Adds support for more complex wheel builds, including:

• data and native_deps (CcInfo) depenencies
• pre_build_hooks, executed before a wheel build that can modify the build environment
• post_build_hooks, executed after a wheel build that can modify the built artifact (e.g., run auditwheel)
• path_tools, a list of tools that are added to the build environment's PATH
• configurable environment variables and PEP 517 config settings

With these features, it becomes possible to build something like psycopg2 which:

1. depends on libpq (from postgres)
2. calls pg_config to learn information about libpq during setup.py invocation

See examples/external_linking/deps for numpy and psycopg2 examples.