language/go: introduce static dependency resolution mode

[blico]

What type of PR is this?

Feature

What package or component does this PR mostly affect?

language/go
repo
go_repository

What does this PR do? Why is it needed?

Creates a new dependencyMode called static, which avoids going out to the network when resolving imports to external labels and instead solely relies on the dependencies Gazelle knows about within WORKSPACE. If no matching dependency is found, Gazelle will skip generating a label for it.

This can greatly improve the performance of go_repository rules by eliminating unnecessary network calls when an external dependency contains importpaths that are not used within the main repository (and therefore not needed).

static is now the default dependencyMode for go_repository rules, given that all known dependencies can be provided via WORKSPACE, and any needed unknown dependencies should already result in build failure.

Which issues(s) does this PR fix?

Fixes #602

Other notes for review
A rough experiment shows in static mode it took a go_repository rule for github.com/docker/docker ~16.5s to fetch, compared to ~633 seconds in external mode:

$ vi WORKSPACE # invalidate rule
$ time bazel fetch @com_github_docker_docker_external//...
Loading: 849 packages loaded

real	10m33.033s
user	0m0.621s
sys	0m0.320s

$ vi WORKSPACE # invalidate rule
$ time bazel fetch @com_github_docker_docker_static//...
Loading: 849 packages loaded

real	0m16.587s
user	0m0.499s
sys	0m0.233s