We can't exactly search the code to find out what it does, otherwise you'd basically be reading all the code to discover it's true behavior, which negates the usefulness of modules in the first place...
Any search will have to rely on developer-made documentation, and/or meta data. This is great in theory, but documentation is rarely well maintained, and/or someone changes the code but neglects to update the documentation.
This leaves us with the situation we have today. A search that somewhat works kindof, and mostly you rely on googling the feature you need and looking for what seems to be the most popular choice.
I'm not sure how this situation can be made better, especially if we continue down a path of having all these "one-liner" libraries/modules that anyone can make and stick out there into the unsuspecting world. When I need a square root function, and my search returns 800 one-liner modules, how am I supposed to pick one that I know is reasonably well done, and does what it says it will do, without looking under the hood - you'll end up just picking whatever seems to be popular...