Support importing Skyframe from a previously dumped file

[benradf]

Currently the tool invokes bazel dump --skyframe for you, but it would be good to also have a mode where you can import from a file that you previously dumped yourself. This could be convenient for extracting a Skyframe dump from an environment where it's awkward to run Skyscope (e.g. if you need to install graphviz etc to get it running). For example:

# On build machine:
bazel dump --skyframe >skyframe.dump

# On developer machine:
skyscope import-skyframe-file skyframe.dump