Feature: Subcommand to create manifest from existing directory

[Gerg-L]

For working around the recursive symlinking problem

[Gerg-L]

We may also want a sub-command to merge manifests... or we can rely on jq. i haven't decided yet

[NotAShelf]

I'd rather not rely on jq. It has correctness and performance issues. jaq is a little better but since we're already using serde & co., I'd rather keep handling this on the Rust side.

[Gerg-L]

should be trivial to implement a merging function. what information to gather from a existing directory will take some deliberation