Make job priority pluggable

[agriyakhetarpal]

Probably we can make job_priority more configurable by user, to make it more useful.

Originally posted by @ryanking13 in #228 (comment)

[agriyakhetarpal]

As discussed in the conversation that followed, this can be utilised to build packages that take a long time to compile either before or last in the build queue. We are still considering how to make this more configurable, but a possible starting point could be pyodide config build_before_all or pyodide config build_after_all, which would accept a string of whitespace-separated package names to be built at the beginning or the end of the queue. We would have to note somewhere that this is only for recipes, and thus is a niche use case for end-users for OOT builds.

[ryanking13]

Maybe we can have something like

[pyodide.build.job_priority]
pkg1 = 1
pkg2 = 2
pkg3 = 3

[agriyakhetarpal]

Yes, that should be fine for an early approach, but I'm not sure if I understand what 1, 2, and 3 mean. Could you please clarify? I am not opposed to it, but I think it would be better to have a more human-readable method for this.

[ryanking13]

My idea was that smaller numbers having higher priority. But I haven't thought about it deeply, and to be honest, I'm not sure about whether this will be useful at all for normal users. Let's think of some more scenarios and discuss them further if they seem useful.