Third-party forks of packaged projects

[Kyle Edwards <kyle.edwards@kitware.com>]

Hello,

I have a question about how Debian handles modifications to third-party 
dependencies. Sometimes a project relies on another project, but has
made modifications to that project that never went into upstream,
either because upstream has abandoned the project or because the
changes are not appropriate for upstream. In that case, the depending
project "vendors" the third-party dependency with the modifications
that it needs.

Obviously, "vendored" dependencies are a no-go in Debian, but how do
situations like this get resolved? I'm imagining the modified project
could be packaged on its own the way any fork would - in that manner,
it would not be much different from packaging MariaDB even though a
package for MySQL already exists. Is my intuition correct here?

Kyle