Invalidate cache when pubspec.lock is modified

[tenhobi]

Add a new prune config (true by default) to invalidate caches when any pubspec.lock is modified since the last build.

That would prevent issues with one generated file using another.

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If any package, which generates code, is updated, build_runner does not recognize that and will use cached version.

This can lead to error where updated dependency assumes different generated code etc.

Runner should cache snapshots of pubspec.lock and allow pruning cache directory if dependencies are updated.