clippy_dev: Parsing revamp part 4/N

[Jarcho]

Based on #15947

Simple change in this one. Lints are parsed into a map indexed by name in order to detect overlapping lint names. The panic message when a duplicate name is parsed is unhelpful right now. That will be fixed in a future PR along with improving all the other parsing errors.

changelog: none

[rustbot]

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[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[Jarcho]

Ping @llogiq.

The fpc label can be ignored. The lint list was just reordered (actually alphabetical for what little that's worth).

[llogiq]

Thanks, merging.