[ffigen] Output dependencies & only rerun if dependencies change

[dcharkes]

Pulling some learnings from #977.

My preference would be to let a higher level of build system handle checking the file times, like Bazel or package:build or native assets.

👍 Great thinking about how this integrates in the larger eco system.

I think the FFIgen (and JNIgen) should be able to report the list of dependencies (#1008). We don't want every user of FFIgen (and JNIgen) to manually come up with a list of dependencies.

The logic of checking the files timestamps could maybe live in a shared package that then FFIgen, JNIgen, and native_assets_builder can reuse.

One question to ask is what abstraction to use for listing dependencies:

1. File paths
2. Directory paths (which would mean all files transitively in that directory, and it also means that if someone adds a file in such directory it's added).
3. Glob patterns? (which would mean that any files added to the file system matching such pattern would trigger a rebuild). -> But not every build system wrapping FFIgen/JNIgen might be able consume glob patterns (native assets, package:build, ...)

Originally posted by @dcharkes in #977 (comment)

Some more edge cases to consider -

1. Transitive dependencies could be changed
2. ffigen version might be updated in pubspec.yaml
3. Any updates to compiler-opts specified via the environment itself.

Originally posted by @mannprerak2 in #977 (comment)

I think the first step is to output a list of dependencies.