c1589b5 commit broke bazel fetch, which makes offline builds impossible.

to reproduce:

• add emsdk http_archive to WORKSPACE,
• execute bazel fetch //... or bazel fetch @<name_given_to_emsdk_dependency>//...

Error produced:

error loading package '@emscripten_toolchain//bazel/hello-world': Label '@emscripten_toolchain//emscripten_toolchain:wasm_rules.bzl' is invalid because 'emscripten_toolchain' is not a package; perhaps you meant to put the colon here: '@emscripten_toolchain//:emscripten_toolchain/wasm_rules.bzl'?

bazel attempts to discover all transient dependencies, but fails when evaluating package names under bazel dir.