repro --pull implies --allow-missing

[dberenbaum]

See https://github.com/iterative/dvc/issues/10412#issuecomment-2103410493. When we introduced --allow-missing, we also updated the behavior of --pull, but kept them as separate flags for a couple reasons (see discussion here):

• didn't want to change behavior of --pull too much
• worried about too much complexity in one flag
• wanted to wait and see how it was used

Since then, every user I have encountered expects the behavior to be what --pull --allow-missing does. I don't see any valid case where someone using --pull would not want --allow-missing (we are basically already implying this by treating missing data as needing to be pulled).

[dberenbaum]

This raised a couple other issues when I started looking deeper:

1. With this PR, --pull may skip unchanged stages without pulling them since --allow-missing is intended to skip these stages entirely. This saves time pulling that data but also may be unexpected/more breaking than if we pull unstaged changes.
2. Regardless of this PR, --allow-missing will skip pull and checkout for unchanged stages, but it will still checkout unchanged stages from the local cache. This may also be expensive, and arguably we should skip checkout for these.

I'm working on an alternate PR that would address these questions.

[dberenbaum]

This raised a couple other issues when I started looking deeper:

1. With this PR, `--pull` may skip unchanged stages without pulling them since `--allow-missing` is intended to skip these stages entirely. This saves time pulling that data but also may be unexpected/more breaking than if we pull unstaged changes.

2. Regardless of this PR, `--allow-missing` will skip pull and checkout for unchanged stages, but it will still checkout unchanged stages from the local cache. This may also be expensive, and arguably we should skip checkout for these.

I'm working on an alternate PR that would address these questions.