clippy_dev refactor

[Jarcho]

Based on #13223

Various refactoring on clippy_dev before upgrading rename and splitting clippy_lints into multiple crates.

Some improvements:

• The working directory is set to the root clippy directory. Running from a subdirectory was kind of supported before sometimes. Now it just works.
• File won't be written unless they're actually updated. Most of the time they weren't written, but a few cases slipped through.
• Buffers are reused more for the negligible speed boost.

changelog: None

[rustbot]

r? @samueltardieu

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[samueltardieu]

Since it's waiting on #13223 and I'll be AFK for a while, reassigning similarly to #13223
r? @flip1995

[Jarcho]

Since @flip1995 is busy r? clippy