Add parameter for Scancode cores

[soimkim]

Description

1. Execute Scancode with multi-processing as many as the number of input cores.

   • Default : Number of cores - 1

2. tox: Use allowlist_externals instead of whitelist_externals

   • whitelist_externals is now deprecated, use allowlist_externals instead. see https://tox.readthedocs.io/en/latest/config.html

3. Fix ScanCode's import LegacyVersion bug by fixing packaging version.

   • ImportError: cannot import name 'LegacyVersion' from 'packaging.version' aboutcode-org/scancode-toolkit#3171

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Documentation update
• Refactoring, Maintenance
• Breaking change (fix or feature that would cause existing functionality to not work as expected)