bazel: support using envoy_build_system.bzl macros from consuming projects. (#709)
bazel: support using envoy_build_system.bzl macros from consuming projects. (#709)

This allows projects that consume Envoy to link in their own filters and write unit/integrations
tests in a similar way to Envoy proper code.

See https://github.com/htuch/envoy-consumer/blob/master/BUILD for how this is used.
sebrandon1/envoy,envoyproxy/envoy,lyft/envoy,tdmackey/envoy,fengli79/envoy,htuch/envoy,craffert0/envoy,istio/envoy,envoyproxy/envoy-wasm,rlazarus/envoy,lyft/envoy,jrajahalme/envoy,jrajahalme/envoy,lizan/envoy,envoyproxy/envoy,istio/envoy,eklitzke/envoy,dnoe/envoy,envoyproxy/envoy-wasm,fengli79/envoy,lizan/envoy,craffert0/envoy,amalgam8/envoy,tschroed/envoy,dnoe/envoy,dnoe/envoy,eklitzke/envoy,lyft/envoy,lizan/envoy,lizan/envoy,timperrett/envoy,eklitzke/envoy,rlazarus/envoy,dnoe/envoy,lyft/envoy,tschroed/envoy,envoyproxy/envoy,timperrett/envoy,timperrett/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy-wasm,craffert0/envoy,amalgam8/envoy,lizan/envoy,htuch/envoy,timperrett/envoy,envoyproxy/envoy,envoyproxy/envoy-wasm,envoyproxy/envoy,rlazarus/envoy,sebrandon1/envoy,sebrandon1/envoy,istio/envoy,amalgam8/envoy,lyft/envoy,lyft/envoy,istio/envoy,envoyproxy/envoy,sebrandon1/envoy,envoyproxy/envoy,envoyproxy/envoy-wasm,eklitzke/envoy,istio/envoy,rlazarus/envoy,fengli79/envoy,jrajahalme/envoy,fengli79/envoy,envoyproxy/envoy,tdmackey/envoy,eklitzke/envoy,envoyproxy/envoy,tdmackey/envoy,envoyproxy/envoy,lizan/envoy,tdmackey/envoy,istio/envoy,envoyproxy/envoy-wasm,dnoe/envoy,istio/envoy,envoyproxy/envoy-wasm,jrajahalme/envoy,craffert0/envoy,tschroed/envoy,eklitzke/envoy,jrajahalme/envoy,amalgam8/envoy,sebrandon1/envoy,jrajahalme/envoy,htuch/envoy