Ensure procmacros can be manually aot-compiled

[mkaput]

Summary

With a naive, no-effort approach, we will have a chicken-and-egg problem here that will make manual building impossible. All because the tarball preparation directory is temporary to the scarb package command execution. To overcome this we have to make sure that it is possible to allow users to include the target/scarb/cairo-plugin directory into the tarball. Cargo has include/exclude fields, which seem like the most plausible solution we can port.

What determines whether package contains prebuilt binaries?

Any Scarb package that contains a target/scarb/cairo-plugin directory in tarball is considered as one containing prebuilt binaries. This name was chosen because it nicely hides inside Scarb’s/Cargo’s standard build file directory, which is very likely to already be avoided in source repositories and thus shouldn’t conflict with package sources.
Within this directory, binaries for all platforms will follow the following naming pattern: ${PACKAGE_NAME}_v${PACKAGE_VERSION}_${TARGET}.${so|dylib|dll}

Platform support

Due to licensing and technical constraints (macOS SDK can only be run on macOS), packaging with AOT compilation will be legally possible only on the following platform:

• aarch64-apple-darwin