"Oh, geez, the first thing that makes code _legacy_ code is people checking in "slightly modified" library/framework code."
For myself, I consider the ability to have a dependency and cleanly put a patch on top of it, such that the tooling can help me when I'm upgrading the dependency, is a basic requirement.
Few, if any, tools seem to consider this a first-class need. Based on what I've seen, I imagine this is because most people tend to prefer putting in a thousand lines in their own code to contort things around to hack the underlying library to mostly do what they need most of the time even if it shreds every abstraction their code is nominally creating rather than making a three line change in the framework itself, even before we're discussing tooling making it harder to patch. Most of the remainder tend to, as you say, hack the library in such a way that it can not be usefully tracked; for this I would suggest the tooling should be considered at fault.
For myself, I consider the ability to have a dependency and cleanly put a patch on top of it, such that the tooling can help me when I'm upgrading the dependency, is a basic requirement.
Few, if any, tools seem to consider this a first-class need. Based on what I've seen, I imagine this is because most people tend to prefer putting in a thousand lines in their own code to contort things around to hack the underlying library to mostly do what they need most of the time even if it shreds every abstraction their code is nominally creating rather than making a three line change in the framework itself, even before we're discussing tooling making it harder to patch. Most of the remainder tend to, as you say, hack the library in such a way that it can not be usefully tracked; for this I would suggest the tooling should be considered at fault.