go_repository: debug mode

[blico]

By design, go_repository rules download and run Gazelle on the entire external repo, even if the main repo only depends on a small portion of that repo. Meaning if Gazelle encounters issues on an unused portion of the repo, it will spam a benign warning whenever the corresponding go_repository rule is invoked.

For example:

$ bazel fetch @com_github_census_instrumentation_opencensus_proto//...
DEBUG: /Users/blico/sources/bazel-gazelle/internal/go_repository.bzl:263:18: com_github_census_instrumentation_opencensus_proto: gazelle: rule //gen-go/agent/metrics/v1:metrics imports "github.com/census-instrumentation/opencensus-proto/gen-go/agent/common/v1" which matches multiple rules: //gen-go/agent/common/v1:common and //src/opencensus/proto/agent/common/v1:common. # gazelle:resolve may be used to disambiguate
gazelle: rule //gen-go/agent/metrics/v1:metrics imports "github.com/census-instrumentation/opencensus-proto/gen-go/metrics/v1" which matches multiple rules: //gen-go/metrics/v1:metrics and //src/opencensus/proto/metrics/v1:metrics. # gazelle:resolve may be used to disambiguate
gazelle: rule //gen-go/agent/metrics/v1:metrics imports "github.com/census-instrumentation/opencensus-proto/gen-go/resource/v1" which matches multiple rules: //gen-go/resource/v1:resource and //src/opencensus/proto/resource/v1:resource. # gazelle:resolve may be used to disambiguate
gazelle: rule //gen-go/agent/trace/v1:trace imports "github.com/census-instrumentation/opencensus-proto/gen-go/agent/common/v1" which matches multiple rules: //gen-go/agent/common/v1:common and //src/opencensus/proto/agent/common/v1:common. # gazelle:resolve may be used to disambiguate
gazelle: rule //gen-go/agent/trace/v1:trace imports "github.com/census-instrumentation/opencensus-proto/gen-go/resource/v1" which matches multiple rules: //gen-go/resource/v1:resource and //src/opencensus/proto/resource/v1:resource. # gazelle:resolve may be used to disambiguate
gazelle: rule //gen-go/agent/trace/v1:trace imports "github.com/census-instrumentation/opencensus-proto/gen-go/trace/v1" which matches multiple rules: //gen-go/trace/v1:trace and //src/opencensus/proto/trace/v1:trace. # gazelle:resolve may be used to disambiguate
gazelle: rule //gen-go/metrics/v1:metrics imports "github.com/census-instrumentation/opencensus-proto/gen-go/resource/v1" which matches multiple rules: //gen-go/resource/v1:resource and //src/opencensus/proto/resource/v1:resource. # gazelle:resolve may be used to disambiguate
gazelle: rule //gen-go/trace/v1:trace imports "github.com/census-instrumentation/opencensus-proto/gen-go/resource/v1" which matches multiple rules: //gen-go/resource/v1:resource and //src/opencensus/proto/resource/v1:resource. # gazelle:resolve may be used to disambiguate

We should suppress this log by default, given that it is only ever helpful when debugging a go_repository rule.

Relevant issues: #947, #859, #576, #1172, #1204