fix: Avoid panic for package specs with an empty fragment

[Pyr0de]

What does this PR try to resolve?

Fixes #16731

PackageIdSpec::parse panics on URL package specs with an empty fragment (...#)
This change would return PackageIdSpecError(ErrorKind::EmptyFragment) instead of panicing

How to test and review this PR?

Earlier the following code would panic

let _ = PackageIdSpec::parse("https://example.com/foo#");

but now it should return an error

[rustbot]

r? @weihanglo

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Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: @ehuss, @epage, @weihanglo
• @ehuss, @epage, @weihanglo expanded to ehuss, epage, weihanglo
• Random selection from ehuss, epage, weihanglo

[0xPoe]

Thanks!

View changes since this review

[0xPoe]

This is a non-public error kind, so I believe this is acceptable.
cc: @weihanglo Not sure if you have any thoughts on it.

[weihanglo]

Yeah should be good

[0xPoe]

Suggested change

	err!("https://crates.io/1foo#1.2.3", ErrorKind::NameValidation(_));
	err!("https://example.com/foo#", ErrorKind::EmptyFragment);
	err!("https://crates.io/1foo#1.2.3", ErrorKind::NameValidation(_));
	err!("https://example.com/foo#", ErrorKind::EmptyFragment);