NF: replace `tree` with python implementation

This implementation mimics the original behavior, while fixing two bugs:

onyo tree now prints from CWD (as documented) rather than repository root. The onyo_tree function still defaults to repository root.
- argument dirs are now printed exactly as the user specified, rather than always absolute

By using pure Python, we gain flexibility to add Onyo-specific features (perhaps asset highlighting, etc) and also drop a dependency (albeit a common one).

We never allowed passing flags to tree, so there really is no loss in functionality.

I did some primitive performance tests.

32 directories, 42 files:
-            tree  0.00s user 0.00s system 87% cpu 0.005 total
- (old) onyo tree  0.09s user 0.01s system 99% cpu 0.094 total
- (new) onyo tree  0.09s user 0.01s system 99% cpu 0.101 total

1844 directories, 9706 files
-            tree  0.06s user 0.03s system 99%  cpu 0.090 total
- (old) onyo tree  0.12s user 0.05s system 106% cpu 0.164 total
- (new) onyo tree  1.21s user 0.08s system 100% cpu 1.289 total

In the "normal" use case, where someone prints a tree of a size that a human can reasonably read, the performance hit is negligible.

However, large trees do suffer from a noticeable performance impact. This can be dropped to 0.43s if all output is printed at once.

1844 directories, 9706 files
- onyo tree  0.38s user 0.07s system 102% cpu 0.435 total

psyinfra / onyo

NF: replace `tree` with python implementation #605

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