This issue proposes an amendment to the Workspace proposal (#45713) to use go.mod file paths rather than directory paths in the directory directive, perhaps renaming the directive to use. It was originally suggested by @ianthehat in an issue comment:

Now this proposal is accepted, I would like to suggest some minor changes.

I think that we should change it from listing directories (and implying the go.mod file in that directory) to explicitly listing the mod file.
I think this would remove some of the confusion about exactly what it means to refer to a directory, and would also allow us to add directories with one of the meanings people have asked for in the future if we wanted to.
The cost is negligible, basically existing lines would just need /go.mod appended to the end, and for the most part those lines are written by a tool anyway.
This approach would also increase the flexibility of the system by allowing the go.work file to refer to module files not called go.mod. The go command already allows you to use this files in normal module mode, the two uses I know the most about are go.tools.mod and gopls.mod, but there are other uses as well. This would allow you for instance to have a tools.work file that referred to the go.tools.mod files instead of the go.mod files.
If we did this, we would also need to change the name of the directive (from directory). I don't really mind what to, I was thinking something like use or include, but suggestions are welcome.
The end result would be (using the same example as the proposal)

go 1.18

use (
    ./baz/go.mod // foo.org/bar/baz
    ./tools/go.mod // golang.org/x/tools
)

replace golang.org/x/net => example.com/fork/net v1.4.5

cc @bcmills @jayconrod