feat: Use SPDX license identifiers when matching package licenses

Match only against the new SPDX license identifiers in the new common licenses location in /usr/share/licenses/spdx/ to figure out if a package needs a custom license file. Also informs the user of unknown license identifiers and suggests to prefix with LicenseRef-. Checks more properly for symlinked files in the same package and counts files to compare against the required amount of custom licenses (based on the license identifiers).

This follows the specification of https://rfc.archlinux.page/0016-spdx-license-identifiers/ which outlines the use of all plaintext SPDX license files for the purpose of a new common licenses package.

cc @foxboron @serebit @jelle