Support vendoring directly into root (workspace_path = "//")

[google]

Support vendoring directly into root (workspace_path = "//")

This would also help with dev tooling. Some editors / IDEs expect the Cargo.toml to be located in the root directory, and break unexpectedly if this is not the case. So to keep tools working, one needs to have both valid Bazel BUILD files and valid Cargo.toml files (at least from the bits of testing I've managed to do thus far).

[acmcarther]

Ack, will work on this tonight.

As noted in #12, the only reason this isn't currently supported is because I was lazy when handling paths, just following a convention of retaining a trailing slash everywhere. "//" is pathological in this simplistic model (in that "///" is invalid, and "/" is also invalid).

@acmcarther thanks for the fast response! If you need help testing or any other simple tasks, please feel free to ping me.

[mfarrugi]

@Zintinio you might want to chime in on bazelbuild/rules_rust#71 as well if your usecase isn't already covered.