Follow-up of https://github.com/zed-industries/zed/pull/13433
Closes https://github.com/zed-industries/zed/issues/27124
Closes https://github.com/zed-industries/zed/issues/27066

After this change, both old global task source, `tasks.json` and new,
`debug.json` started to call for the same task update method:


https://github.com/zed-industries/zed/blob/14920ab910c6d0208d23ce6b6e2ed644e6f20f2e/crates/project/src/task_inventory.rs#L414

erasing previous declarations.

The PR puts this data under different paths instead and adjusts the code
around it.

Release Notes:

- Fixed custom tasks not shown