Static compilation and `*_jll` packages is not relocatable

[apozharski]

It would be very useful if the binary wrapping packages built by *_jll were relocatable. Currently the artifact path is determined at compile time in these lines:

        function find_artifact_dir()
            # We determine at compile-time whether our JLL package has been dev'ed and overridden
            @static if isdir(joinpath(dirname($(pkg_dir)), "override"))
                return joinpath(dirname($(pkg_dir)), "override")
            elseif @isdefined(augment_platform!) && VERSION >= v"1.6"
                $(Expr(:macrocall, Symbol("@artifact_str"), __source__, src_name, __module__.host_platform))
            else
                # We explicitly use `macrocall` here so that we can manually pass the `__source__`
                # argument, to avoid `@artifact_str` trying to lookup `Artifacts.toml` here.
                return $(Expr(:macrocall, Symbol("@artifact_str"), __source__, src_name))
            end
        end

Why: My current use case is to build a C interface for a package (namely MadNLP.jl) which pulls in HSL_jll the publicly available form of which is just a shim with no actual functionality. As such the end user (in an ideal case) would have to provide their own version of HSL_jll and an Overrides.toml that points to it. However because the overrides are determined at compile-time, the paths and overrides are compiled statically into the binaries and there is no way for a user to point these to a different place after the fact.

I do not entirely understand at the moment if there are some major performance implications to changing the code to determining the artifact directory at e.g. __init__() time, but doing so would greatly improve the relocatability of bundles which include *_jll packages.