[8.0.1] Streamline `ModuleExtensionId#toString()` & Fix edge cases in lockfile handling (#24845)

### Streamline `ModuleExtensionId#toString()`

Currently `ModuleExtensionId#toString()` uses the default Java record
repr, which isn't very user-friendly. There's really no reason not to
use the `asTargetString()` method instead. This PR just renames
`asTargetString()` to `toString()` and updates all call sites.

Closes #24450.

PiperOrigin-RevId: 700050362
Change-Id: I82238d2134e1642694f0b20235fcfe9307ceaa7d 
(cherry picked from commit d23421a0)

### Fix edge cases in lockfile handling

* Don't run the core logic when `--lockfile_mode` is `off` or `error`
but the command doesn't forward options to Skyframe.
* Honor `reproducible` per extension eval factor, not per extension.
* Fix encoding conflict between isolation key and `use_repo_rule`'s fake
extension names

This doesn't require a lockfile version bump as `use_repo_rule`'s fake
extension (so far) isn't included in the lockfile.

Work towards #24723

Closes #24754.

PiperOrigin-RevId: 712623562
Change-Id: I61fd439539031a01ddec4488276ff2d0484849f2 
(cherry picked from commit cfda1780

)

Fixes #24754

Co-authored-by: Xdng Yng <wyverald@gmail.com>