Many languages' package management systems install third-party packages in a subfolder of a given project's root repository. (E.g. node_modules/, deps/, etc.) This is nice as it allows multiple versions of a single package to coexist nicely, as each is isolated to the project that depends on it.
raco by default installs packages system-wide it seems. Is there a way to tell raco it should install packages in a particular folder?
The comment is right to point you to package scopes: using a directory package scope is the most fine-grained level of control available at the level of the package manager. However, as the docs say,
Conflict checking disallows installation of the same or conflicting package in different scopes …
It is a design decision in the Racket package system not to allow multiple versions of the same package to coexist in the same installation. The idea is that a package shouldn't make breaking changes while using the same name. One inspiration that's been mentioned is OS level package systems like Debian's, where gcc-4.7 and gcc-4.8 are different packages that install different binaries. This is very different than the practice of some other language package systems (e.g. npm), but it works out well in practice because the Racket community takes backwards-compatibility quite seriously.
This is actually Racket's second package system. The old system, PLaneT, tried to be much more opinionated, including built-in notions of versioning. The new/current system consciously removed some of those elements to create a more minimal and focused package system, which means that a great deal of what formerly had to be "built in" to Racket can now be just another package. You may be interested in a talk about the design of the package system. There was also a fair amount of discussion at the time on the mailing list, which is archived here.
If you do find you need different package versions to an extent that can't be supported by existing mechanisms, you could use a stand-alone Racket installation (rather than a system-wide one) to get a completely isolated environment. I'm sure the Racket community would also be interested in hearing about any issues you have in practice.