Case insensitive file name matching when packaging files

[torhovland]

Fixes #13722.

Handles the readme and license files. The Cargo.lock file is trickier. There are a bunch of places in the code where the existence of Cargo.lock (case sensitive) is checked, including when packaging.

I think it should be OK to keep Cargo.lock out of this. While the readme and license files are typically created by the user, the Cargo.lock file is typically created by Cargo, hence casing should be consistent anyway.

This PR also doesn't handle a TARGET dir. I can look into that if desired.

#13722 (comment)

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @epage (or someone else) some time within the next two weeks.

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[epage]

Note that #13722 is not marked as S-Accepted. I don't think we've come to a conclusion on if it should be fixed and what the right constraints are.

[torhovland]

I am now noticing that Cargo uses the unicase crate for case insensitive comparisons elsewhere.

use unicase::Ascii as UncasedAscii;

This PR should do that rather than explicitly converting to lowercase and comparing.

[weihanglo]

Thanks for the PR. Marked it as draft for now to reflect the fact that it's still in discussion.