fix(pkg): resolve symlinks from (install) action

[Alizter]

The reloctable compiler, like the other OCaml compilers, has symlinks in it's installation layout. These need to be resolved before we declare them inside the directory target or else the reloctable compiler cannot be cached. This work is tracked in #13229.

In the second commit we introduce a general mechanism to comb through the targets of the install layout for a package and resolve these symlinks by replacing them with a hardlink, falling back to a copy if that is not possible.

We add quite a few tests to exercise the numerous edge cases that come up.

• TODO describe tests a bit more

[rgrinberg]

Here's a suggestion to improve the readability:

Instead of adding a run for every single command, why not a single bash action with a multi line string? They were made for exactly this purpose.

[Alizter]

We don't support (bash) in lock files.

[rgrinberg]

Okay. Use sh

[Alizter]

Do you want them in a single sh -c or line by line?

[rgrinberg]

If it was line by line, I think it would defeat the purpose of the change. Since you're already using the shell, use it all the way and invoke it only once do all the work.

[rgrinberg]

Similarly, in the actions where you don't use the shell (I think there's a couple?) you can leave things as is and use the dune language for actions.