release-24.1: schemachanger: consider column target status after resolving

[blathers-crl]

Backport 2/2 commits from #122389 on behalf of @annrpom.

/cc @cockroachdb/release

---

schemachanger: consider "dropping" columns during drop col

Previously, using the "comma syntax" for dropping a col
with the same name (alter table t drop column i, drop column i;)
led to an internal error. This patch makes it so that
the error will instead be a proper external error by
ensuring that resolving columns in the DSC considers the
"dropping" state.

Epic: none

Release note: None

---

schemachanger: consider "dropping" columns while adding col

This patch enables the "comma syntax" to drop and add
a column with the same name in our DSC.

Epic: none
Fixes: #120226

Release note (bug fix): Dropping and adding a column with the same
name no longer results in a "column already exists error".

---

Release justification: bug fix

[blathers-crl]

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[blathers-crl]

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[cockroach-teamcity]

This change is 

[fqazi]

Reviewed 1 of 1 files at r1, 17 of 17 files at r2, all commit messages.
Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @annrpom and @rafiss)

[annrpom]

getting Required statuses must pass before merging -- i'll try rebasing

edit: i force-pushed to this branch as per https://cockroachlabs.slack.com/archives/CJ0H8Q97C/p1648648510522509?thread_ts=1648638969.754059&cid=CJ0H8Q97C