[ffigen] Output dependencies

[dcharkes]

If FFIgen is run as part of another build system (#977 (comment)), or if FFIgen is run as standalone (#879), it would be useful to know what files an FFIgen run transitively depends on.

The list of files / directories / glob patterns should include:

• entrypoints headers declared in config.
• Any transitive dependencies (can libclang report what was used? Would using include directive suffice?)
• pubspec.yaml ffigen version might be updated
• ffigen.yaml the config file

For standalone runs:

• We should maybe save the environment, so that subsequent runs can check for equality.

For runs from another build system:

• We should maybe have a flag in the config to ignore the environment (A build system invoking might also try to invoke FFIgen with a (mostly) empty environment [native_assets_cli] Protocol environment variables #32)

The list of files/directories/globs should be output in some kind of format. (Maybe we should align with the BuildOutput.dependencies in the native assets CLI, which currently supports files and directories.)

Edit: This would tie in to dart-lang/sdk#56512.