This makes it slightly more convenient to use the following as a
Reindeer fixup for those that prefer to build from source:
extra_mapped_srcs = { "src/lib_from_source.rs" = "src/lib.rs" }
[platform_fixups.'cfg(all(target_arch = "x86_64", target_os = "linux", target_env = "gnu"))']
extra_deps = [":proc-macro2", ":quote", ":syn"]
as opposed to checking in a whole new file containing the `extern crate
proc_macro` + `include!("lib_from_source.rs")`.
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