RFC: A better error message for uncapitalized package name

Making this issue per discussion here. I was trying to install Duper and got the following error which confused me:

boltonbailey@starlight drafting % lake update duper
warning: drafting: package 'Duper' was required as 'duper'
mathlib: running post-update hooks
error: dependency 'duper' not in manifest; use `lake update duper` to add it
error: mathlib: failed to fetch cache
boltonbailey@starlight drafting % lake update duper
warning: drafting: package 'Duper' was required as 'duper'
mathlib: running post-update hooks
error: dependency 'duper' not in manifest; use `lake update duper` to add it
error: mathlib: failed to fetch cache
boltonbailey@starlight drafting %

I was mainly confused by the fact that the error told me to run the same command I had just run, but potentially another thing that would have made things clear to me would have been a suggestion that my require statement was uncapitalized, when it should have been capitalized (this was what fixed it).

Reproduce the reported lake update duper output with an uncapitalized package name, then trace the diagnostic around the require statement and package-name handling. Done means the error no longer repeats the same command without explanation and clearly identifies the capitalization correction.