If project uses C and does not include `stdbool.h`,
it will not have available lowercase booleans.
However, since this tool targets GLib programs,
we can rely on uppercase constants defined by GLib.
evolution-ews introduced a check for optional schemas, which would skip our hardcoded code paths.
a0c514bd34
Let’s update the semantic patch to also replace the invocations of specified `schemaExistsFunction` for known schemas with `true`.
glib expression is messy enough as is.
Also rename the `glib-schema-to-var` argument to `schemaIdToVariableMapping` to better match Nixpkgs coding style.
