RFC: Display and use SPDX licenses ids throughout

[pombredanne]

As SBOMs start to get some adoption, we should consider using SPDX license identifiers for display and reporting throughout the AboutCode stack and demote our own license keys as secondary.

• DejaCode: Use SPDX license ids first, everywhere
• ScanCode.io: Use SPDX license ids first, everywhere
• PurlDB: Use SPDX license ids first, everywhere
• ScanCode Toolkit: Use SPDX license ids first, everywhere
• inspectors: Use SPDX license ids first, everywhere