[CI] Refactor version comparison logic in `check_versions` job

[joshlf]

In CI, we have the following job:

zerocopy/.github/workflows/ci.yml

Lines 334 to 408 in 3bb9a54

	check_versions:
	needs: generate_cache
	runs-on: ubuntu-latest
	name: Check crate versions match
	steps:
	- uses: actions/checkout@8ade135a41bc03ea155e62e844d188df1ea18608 # v4.1.0
	- uses: actions/cache@v3
	with:
	path: |
	~/.cargo/
	key: ${{ runner.os }}-cargo-${{ hashFiles('**/Cargo.toml') }}
	# Make sure that both crates are at the same version, and that zerocopy
	# depends exactly upon the current version of zerocopy-derive. See
	# `INTERNAL.md` for an explanation of why we do this.
	- name: Check crate versions match
	run: |
	set -eo pipefail
	# Usage: version <crate-name>
	function version {
	cargo metadata --format-version 1 | jq -r ".packages[] | select(.name == \"$1\").version"
	}
	ver_zerocopy=$(version zerocopy)
	ver_zerocopy_derive=$(version zerocopy-derive)
	# The non-dev dependency version (`.kind == null` filters out the dev
	# dependency, and `.target == null` filters out the targeted version).
	zerocopy_derive_dep_ver=$(cargo metadata --format-version 1 \
	| jq -r ".packages[] | select(.name == \"zerocopy\").dependencies[] | select((.name == \"zerocopy-derive\") and .kind == null and .target == null).req")
	# The non-dev dependency, targeted version.
	zerocopy_derive_targeted_ver=$(cargo metadata --format-version 1 \
	| jq -r ".packages[] | select(.name == \"zerocopy\").dependencies[] | select((.name == \"zerocopy-derive\") and .kind == null and .target == \"cfg(any())\").req")
	# The dev dependency version (`.kind == \"dev\"` selects only the dev
	# dependency).
	zerocopy_derive_dev_dep_ver=$(cargo metadata --format-version 1 \
	| jq -r ".packages[] | select(.name == \"zerocopy\").dependencies[] | select((.name == \"zerocopy-derive\") and .kind == \"dev\").req")
	if [[ "$ver_zerocopy" == "$ver_zerocopy_derive" ]]; then
	echo "Same crate version ($ver_zerocopy) found for zerocopy and zerocopy-derive." | tee -a $GITHUB_STEP_SUMMARY
	else
	echo "Different crate versions found for zerocopy ($ver_zerocopy) and zerocopy-derive ($ver_zerocopy_derive)." \
	| tee -a $GITHUB_STEP_SUMMARY >&2
	exit 1
	fi
	# Note the leading `=` sign - the dependency needs to be an exact one.
	if [[ "=$ver_zerocopy_derive" == "$zerocopy_derive_dep_ver" ]]; then
	echo "zerocopy depends upon same version of zerocopy-derive in-tree ($zerocopy_derive_dep_ver)." | tee -a $GITHUB_STEP_SUMMARY
	else
	echo "zerocopy depends upon different version of zerocopy-derive ($zerocopy_derive_dep_ver) than the one in-tree ($ver_zerocopy_derive)." \
	| tee -a $GITHUB_STEP_SUMMARY >&2
	exit 1
	fi
	if [[ "=$ver_zerocopy_derive" == "$zerocopy_derive_dev_dep_ver" ]]; then
	echo "In dev mode, zerocopy depends upon same version of zerocopy-derive in-tree ($zerocopy_derive_dev_dep_ver)." | tee -a $GITHUB_STEP_SUMMARY
	else
	echo "In dev mode, zerocopy depends upon different version of zerocopy-derive ($zerocopy_derive_dev_dep_ver) than the one in-tree ($ver_zerocopy_derive)." \
	| tee -a $GITHUB_STEP_SUMMARY >&2
	exit 1
	fi
	if [[ "$zerocopy_derive_dep_ver" == "$zerocopy_derive_targeted_ver" ]]; then
	echo "Same crate version ($zerocopy_derive_dep_ver) found for optional and targeted zerocopy-derive dependency." \
	| tee -a $GITHUB_STEP_SUMMARY
	else
	echo "Different crate versions found for optional ($zerocopy_derive_dep_ver) and targeted ($zerocopy_derive_targeted_ver) dependency." \
	| tee -a $GITHUB_STEP_SUMMARY >&2
	exit 1
	fi

Over time, it has grown to contain a lot of duplicated code. The task for this issue is to deduplicate where it makes sense to do so using whatever techniques are appropriate (e.g., bash functions, etc).