validate shape of `packages.config` and consolidate error handling #28617

Job	Run time
ci (bundler, bundler, bundler)	16s
integration	9m 8s
ci (cargo, cargo, cargo)	12s
ci (common, common, common)	1m 0s
ci (composer, composer, composer)	12s
Lint	1m 1s
ci (docker, docker, docker)	18s
Updater	3m 2s
ci (dotnet_sdk, dotnet_sdk, dotnet-sdk)	11s
ci (elm, elm, elm)	11s
ci (git_submodules, git_submodules, gitsubmodule)	10s
ci (github_actions, github_actions, github-actions)	11s
ci (go_modules, go_module, gomod)	10s
ci (gradle, gradle, gradle)	10s
ci (hex, hex, mix)	16s
ci (maven, maven, maven)	17s
ci (npm_and_yarn, npm_and_yarn, npm)	11s
ci (nuget, nuget, nuget)	20m 58s
ci (pub, pub, pub)	12s
ci (python, python, pip)	16s
ci (python, python_slow, pip)	16s
ci (swift, swift, swift)	11s
ci (devcontainers, devcontainers, devcontainers)	16s
ci (terraform, terraform, terraform)	10s
	39m 15s

Provide feedback