Fixes #8770

Problem

The dependencyLock task writes deps.lock as a single line of JSON, which is difficult to review and diff (see #8770).

Solution

Use PrettyPrinter instead of CompactPrinter in LockFile.write so the lock file is written as pretty-printed, multi-line JSON. Schema and semantics are unchanged; only formatting changes. Both the manual dependencyLock task and the lock file written during resolution use this path.

Testing

• Reproduced the issue: ran dependencyLock on sbt 2.0.0-RC9 and confirmed one-line output before the change.
• Added unit test in LockFileSpec: "LockFile.write outputs pretty JSON" (asserts written content contains newlines).
• Existing lock round-trip and dependencyLockCheck behavior unchanged; parser accepts both compact and pretty JSON.

Checklist

• Reproduced the problem before changing code.
• Change compiles and fixes the problem.
• Unit test added.
• Small, focused change (formatting only).