SPDX License Identifiers are a more compact / standardized
way of representing file license information.
See: https://spdx.dev/resources/use/#identifiers
This was done with the `ambr` search and replace tool.
ambr --no-parent-ignore --key-from-file --rep-from-file key.txt rep.txt *
We can't always rely on the initial URL's path(), so use either the
user-specified argument or the URL path for determining the realpath,
depending on whether we got a file:// URL argument.
Fixes#4950.
