Merge utilities package

[mattwthompson]

Summary of changes

• Conda/AmberTools provenance methods are now in openff.toolkit.utils._provenance, intentionally private
• skip_if_missing, skip_if_missing_exec, and requires_pkg (for testing purposes) are now found in openff.toolkit._tests.utils
• requires_package (for non-testing purposes), requires_oe_module, has_package, its alias has_pkg, and has_executable are now found in openff.toolkit.utils.utils
• MissingOptionalDependencyError, CondaExecutableNotFoundError, and CondaExecutableNotFoundWarning are now found in openff.toolkit.utils.exceptions just like all other exceptions and warnings.
  • The above exceptions now inherit from OpenFFToolkitException instead of a OpenFFError which exists in openff.utilities but is probably un-noticed by the world
• get_data_dir_path, similar to an existing get_data_file_path, is added to openff.toolkit.utils.utils

• Consolidate duplicate code paths
• Update some re-exports
• Add tests
• Update docstrings/documentation, if applicable
• Lint codebase
• Update changelog

[codecov]

Codecov Report

❌ Patch coverage is 50.92593% with 53 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.47%. Comparing base (f4566b8) to head (d671782).

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