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 *
This way you don't have to look at all the library names if you don't
want to. Since we're pretty good about namespacing our things, the
library names are slightly redundant information.
