When a dependency gets fetched from Git, it's possible for its name to
contain slashes - e.g. because the dependency's branch name contains
one:
```
something-something = { git = "...", branch = "JIRA-123/dev" }
```
Currently, because we don't sanitize those slashes, trying to compile
such `Cargo.toml` / `Cargo.lock` would fail - e.g. given a dependency
from branch `JIRA-123/dev`, we'd try to extract it into path
`JIRA-123/dev` instead of something flat such as `JIRA-123_dev`.
This commit fixes this issue by sanitizing all the `nkey`s - basically,
for all dependency paths, we replace `/` with `_`.
The choice of underscore is arbitrary, just seemed like a good fit.
The git dependencies were indexed by crate name, which is an issue if a dependency is imported through git at various commits. Moreover, if a crate is imported several time at _the same commit_, it is now unpacked only once.
Otherwise builds fail with an error like:
Compiling simple-dep v0.1.0 (/private/var/folders/5h/b11m6fxj2tqgbxwz5bgjy42c0000gn/T/nix-build-some-name.drv-0/dummy-src)
error: couldn't read build.rs: No such file or directory (os error 2)
