api: support find.Network by cluster path

[dougm]

Description

Datacenter's network folder may contain items with the same name.
For clusters that have one such instance of a given network name, the cluster inventory path will be unique.

Example:

% govc ls -l -t DistributedVirtualPortgroup /DC/host/compute-cluster
/DC/host/dev-cluster/vnet-domain-c9:f6555e6e-051f-4eec-9275-78ba7f689335-test-gc-e2e-demo-ns-91610-0 (DistributedVirtualPortgroup)
/DC/host/dev-cluster/seg-domain-c9:f6555e6e-051f-4eec-9275-78ba7f689335-test-gc-e2e-demo-ns-0 (DistributedVirtualPortgroup)
/DC/host/dev-cluster/vm-domain-c9:f6555e6e-051f-4eec-9275-78ba7f689335-vif-default-network-0 (DistributedVirtualPortgroup)
/DC/host/dev-cluster/containerswitchavi-1 (DistributedVirtualPortgroup)
/DC/host/dev-cluster/containerswitch-1 (DistributedVirtualPortgroup)
/DC/host/dev-cluster/dvpg-vmotion-1 (DistributedVirtualPortgroup)
/DC/host/dev-cluster/wcp_vds_1-DVUplinks-34 (DistributedVirtualPortgroup)

api: support find.NetworkList by ID

Rather than limit by ID search to find.Network, move the logic to NetworkList to support both methods.

Add finder_test (meant to include with PR #2626)

Type of change

Please mark options that are relevant:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to
  not work as expected)
• This change requires a documentation update
• Build related change

[tenthirtyam]

LGTM! 🚀