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 *
Now that we've moved to atomic replacement of these files when altering
them, we don't need to keep them open for the lifetime of Core::Account
so just simplify this and close them when they are not needed.
