It seems `requirementslib` is causing a crash in `pipenv` in some rare circumstances. If you define a package like this in a `Pipfile`:
```
myproject = {editable = true, path = "."}
```
and this project path contains an at sign (`@`), then requirementslib will parse the path wrong. The second clause in the added test case will fail without this fix.

Fixes sarugaku#309

The motivation is to only care about @ that is not part of the path but rather used for git refs