Pidlock source and artifact installation

[IanButterworth]

Fixes #3495
Fixes #4174
Also fixes registry installation which wasn't checking after acquiring the pidlock which registries still needed installing.
Makes progress on #2219

But only if all concurrent julia process installations have this fix.. I'm not sure there's a way to protect against older versions..

Source:

• Pidlocks the source path during check/download/installation
• Protects against an interrupted mv from temp dir to final path by checking tree_hash before and after and deleting the destination if bad.

Artifacts:

• Pidlocks artifact downloads and updates the progress meter to indicate another process is doing the downloading

Registries:

• Checks after acquiring the lock whether it still needs installing

Screen.Recording.2025-02-25.at.5.20.45.PM.mov

Todo:

• Disable test debug prints
• Decide how much testing to keep - One test with 3 concurrent processes

[IanButterworth]

These tests add between 1m40s and 4m (linux git registry).

I'm thinking of merging this and trying out in a Pkg bump PR to see the impact on Julia CI. Adjusting as necessary.

[IanButterworth]

I saw the same hang locally on macOS when testing two concurrent processes, so I don't think the test failures are just CI peculiarities.

https://github.com/user-attachments/assets/8764844b-cfd2-4299-9b0e-cbb0056055db

Update: Fixed. I had a design flaw not pushing the expected number of elements to a channel.
It's not clear to me exactly why it wasn't a problem on some platforms.. I guess some less powerful CI machines had processes that just didn't advance enough and avoided the logic.. Anyway it seems to have fixed it.

[IanButterworth]

@staticfloat this should be good to go now

I want to test this on Julia CI so I'm going to dismiss these requested changes (which have been addressed).

Post merge review very much welcome!