repro: make stages run in order of the graph

[skshetry]

This PR closes #9367, and makes a few other changes:

• --single-item now runs in the order the target is provided, instead of the position in it's dag. So, if you provide dvc repro stage1 stage2 --single-item, stage1 will run before stage2 even if stage2 comes before stage1. Because that's what you want most of the time.
  (And I did not want to complicate the code too much.)
• --downstream stages will run in the order of the graph instead of running stages of each target with post-order traversal.

The order is also enforced in normal cases, not just in --downstream by traversing the graph in post order. Previously, we were removing duplicates manually.

dvc/dvc/repo/reproduce.py

Lines 222 to 228 in 36ec25f

	for stage in all_pipelines:
	if stage not in steps:
	# NOTE: order of steps still matters for single_item
	if single_item and stage not in stages:
	continue
	steps.append(stage)

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.06 ⚠️

Comparison is base (36ec25f) 90.63% compared to head (3834cb8) 90.57%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9675      +/-   ##
==========================================
- Coverage   90.63%   90.57%   -0.06%     
==========================================
  Files         472      480       +8     
  Lines       36280    36334      +54     
  Branches     5218     5219       +1     
==========================================
+ Hits        32882    32911      +29     
- Misses       2812     2839      +27     
+ Partials      586      584       -2     

Impacted Files	Coverage Δ
dvc/repo/graph.py	92.59% <100.00%> (+1.54%)	⬆️
dvc/repo/reproduce.py	98.97% <100.00%> (+2.71%)	⬆️
tests/func/repro/test_repro.py	100.00% <100.00%> (ø)
tests/unit/repo/test_graph.py	100.00% <100.00%> (ø)

... and 9 files with indirect coverage changes

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[shcheklein]

Let's check and update the docs for this please. I think we had one or more descriptions of the execution order. Plus we need to check the option descriptions.

[skshetry]

Nothing should change from the user’s perspective here, at least from what’s documented. This is mostly a bugfix.

[shcheklein]

Asair we mention that we don't guarantee the order of execution, i might be wrong though.