Not the GP, but I note it's difficult to, say, take some code which uses package xyz and substitute the source-compatible package abc without editing every file

Not the GP, but I note it's difficult to, say, take some code which uses package xyz and substitute the source-compatible package abc without editing every file. e.g. Try compiling a large project with a custom version of 'os' swapped in. (e.g. To add tracing, or simulate random I/O failures.) Some package management systems can decouple what packages are called inside the code from what they're called outside.