Add a way to patch `Base.pkgorigins` at compile time

[JanisErdmanis]

Base.pkgorigins is useful for referencing non-Julia assets that live inside a package directory — e.g. pkgdir(@__MODULE__)/assets/index.html — in a relocatable way. This works out of the box with pkgimages.

With sysimages it does not: the recorded paths point at the build machine. The usual remedy is to patch Base.pkgorigins, as the Dyad distribution does in patch-pkgorigins-paths.jl.

JuliaC currently exposes no API for applying such a patch at compile time. To get consistent behaviour across platforms, AppBundler instead patches at runtime from @main:

import AppEnv

function (@main)(ARGS)
    AppEnv.init()
    # application code
    return 0
end

This works, but has drawbacks:

• Stale paths ship anyway. Patching in memory leaves the build-machine strings in the binary, where strings finds them.
• Opt-in and silent when forgotten. AppEnv.init() must run first; miss it and asset lookups quietly point at a nonexistent path.
• Adds an AppEnv dependency to the application project.

Compile-time patching seems preferable. Besides being simpler, it also avoids leaking build paths into the shipped artifact — something the Dyad team raised as a concern, since those paths can expose details of their CI infrastructure.

Would an API for this be in scope for JuliaC?