validate shape of `packages.config` and consolidate error handling #28588

Job	Run time
ci (bundler, bundler, bundler)	11s
Lint	59s
ci (cargo, cargo, cargo)	10s
integration	3m 18s
ci (common, common, common)	51s
Updater	2m 55s
ci (composer, composer, composer)	11s
ci (docker, docker, docker)	15s
ci (dotnet_sdk, dotnet_sdk, dotnet-sdk)	11s
ci (elm, elm, elm)	10s
ci (git_submodules, git_submodules, gitsubmodule)	15s
ci (github_actions, github_actions, github-actions)	11s
ci (go_modules, go_module, gomod)	9s
ci (gradle, gradle, gradle)	11s
ci (hex, hex, mix)	15s
ci (maven, maven, maven)	12s
ci (npm_and_yarn, npm_and_yarn, npm)	11s
ci (nuget, nuget, nuget)	20m 50s
ci (pub, pub, pub)	16s
ci (python, python, pip)	16s
ci (python, python_slow, pip)	10s
ci (swift, swift, swift)	9s
ci (devcontainers, devcontainers, devcontainers)	15s
ci (terraform, terraform, terraform)	10s
	32m 41s

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