So, what can be done?
Simple, isn't it? And it doesn't have to be too complicated. In fact, creating such unified files can be done by hand or by deployment script, and it is definitely an option. However, if you update your site once in 2 years, and then you happen to change one of the individual files but fail to recreate the combined one, strange things might happen.
Things to consider
One of the things you probably should think about is the order of invocation. In your original HTML page the files were loaded in the same order they were listed (unless you played tricks with "defer" attribute - not very portable!). However, the order in which your files will be scanned on the server might be different! So the best approach would be not to have any dependency between the files or, alternatively, to list them on the server (in appropriate configuration place) in the right order.
The important bit is, of course, how far we take it. The following ideas are given to the curious reader as an exercise to implement in his/her programming language of choice:
- Aggressive caching (not validating the underlying files for some period or number of requests)
- Support for If-Modified-Since request header
- Support for compressed output (Accept-encoding request headers), with optional pre-caching of compressed output