End-user is, I think, a very clear term with hindsight, although it is much harder with foresight. In simple terms, the end-user is the person who will be utilising the facilities that you provide. And, as others have mentioned, there can be multiple end-users for facilities being provided to a development environment. In fact, there are often multiple end-users, because different people have to use different parts of software written.
So for a ui library, the end users are the developers who have to use them - they should have a development facility, and this require end-user work for a developer. There are also, possibly, systems people and developers who may have to install the facilities - these are also end-users for the installation tools provided.
And there are the people who use the interface when it is written, who are also end-users. And these may be different types of users - those who produce layouts, for example, and those who display the results.
So I think you need to consider that you have a range of interfaces with your tools, and each of these is an end-user for an interface. Each needs to be considered as a different type of end-user, and designed for explicitly.
What is the point in the term? Well it serves, I think, to focus developers on the person who will eventually be using the code - as opposed to the testers or the immediate client ( who may be defining some of the functionality ). It means that we sometimes focus on people we might not meet, but who will be using the facilities regularly and repeatedly. If we get it right for them, then what we do will be making a positive difference to peoples computer usage.
The installation interface is probably the most interesting one to look at, in some ways. If software installs easily and smoothly, without causing problems, without demanding other software, without breaking anything else, without doing an Adobe and insisting on being updated every day, then nobody will comment on the installation, and the installation end-users wil be happy, meaning that the UI is correct and working. The end-users are generally technical people, but who want to run the installation and move on, nad never have to worry about it again. So design the installation around these end users.