Add workaround for musl lack of SONAME support

[staticfloat]

Because musl doesn't pay attention to SONAMEs, we cannot load JLLs
with transient dependencies because musl always tries to do an
environment search for dependencies, even if they're already loaded. We
work around this by manually altering the dso structures that musl
maintains for each loaded library, making it look like the library was
loaded by a previous environment search instead of by loading directly
via full path.

[giordano]

At this point we can do #31 as well? 🙃

[staticfloat]

Heh, sure. We can do both for now.

[giordano]

CC: @jpsamaroo

[staticfloat]

With this I can now do:

$ docker run -ti julia:alpine
(@v1.6) pkg> add JLLWrappers#sf/musl_workaround
(@v1.6) pkg> add Tar_jll
julia> using Tar_jll

[staticfloat]

@vtjnash I'm pinging you because I think we arguably want to do this for all dlopen()'s in Julia. What do you think?

[jpsamaroo]

This is a pretty hilarious hack... do we know if it has a potential to break in the future?

[giordano]

A few minutes ago I wrote this on Slack to Elliot 😛

[staticfloat]

Just for the record, this only works on Alpine 3.11+, before that the layout is different, so unless I add some code to deal with that, we will indeed have issues around that.

[staticfloat]

Okay, this now passes testing on all versions of Alpine from 3.6+. I defined layouts for versions of musl earlier than that, but Alpine backports lots of patches that change the layout of the DSO object on those earlier versions (but still identifies the musl version as the old version), so it's a losing battle to try to support those old versions. God willing nobody is using such old versions anyway.

This is an annoying amount of copypasta. @vtjnash what are your thoughts on this? Is this a doomed approach? Should we instead have Julia notice it's running on musl, then re-invoke itself with a custom interpreter that has Julian's patch applied?