[SAoC 2021] Solve Dependency Hell: Milestone 1, Week 2

[Ahmet Sait]

[Milestone 
Overview](https://gist.github.com/ahmetsait/10925a246b0d412a698e927658f3f363#milestone-overview)

Hi,
This week I looked into implementing `-mangle-prefix` semantics. 
There is a [draft PR](https://github.com/dlang/dmd/pull/13115) 
(reviews are welcome!) which you can check out for a bit more 
detail. So far I managed to figure out how to properly mangle 
most symbols except auto-generated symbols such as `ModuleInfo`.

Next week I'll be looking into addressing PR reviews, adding 
coverage tests, as well as figuring out how to properly handle 
`ModuleInfo`.