[fidl][net] Rename net_stack

Now that BLD-175 is fixed, rename the stack library to
fuchsia.net.stack.

NET-1194
Test: builds and runs, manually tested with corresponding garnet change
and `run net-cli`

Change-Id: Ifa9c68e9c4bbc5069e6c48bbba473b4b13e0f9b7

[fidl] Rename FIDL directories

These names more closely match what we publish in the SDK. Ideally, we'd
use . as the separator, but Zircon's build system cannot handle . in
directory names, so we use - instead.

Test: Builds.
Change-Id: Ia420a364d2accac39faebade8f3f8ed72088556a