builder/googlecompute: Derive network and subnetwork IDs locally
builder/googlecompute: Derive network and subnetwork IDs locally

This change constructs partial URLs for networks and subnetworks if they
are not already partial or full URLs (i.e., they do not contain a '/' in
their name). Network and subnetwork self-links are no longer retrieved
from the API.

Previously, if a user did not provide the network or subnetwork as a
fully-qualified URL (i.e., self-link), the builder would make
compute.(sub)networks.get API calls with the provided identifier to
discover the self-link. This requires the user or service account Packer
is using to have permission to describe those network resources, which
is becoming less common as IAM is used more. Specifically, a user may
have permission to launch a VM into a network/subnetwork, but will not
have permission to call APIs to describe network resources.

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