Housekeeping (#8)

* Rename the provider to dockerbuild.
* Add Makefile targets required by CI.
* Add per-language test targets compatible with CI.
* Fix broken example yaml.
* Add gitignore exclusions to fix SDK generation in CI.
* Fix lint errors.
* Vendor pulumi CLI via `tools.go` instead of `.pulumi`.
* Consolidate `go.mod`s into one file, with a minimal `go.mod` for the
Go SDK.
* Add codecov.

examples/dotnet/Pulumi.yaml

@@ -1,6 +1,6 @@
-name: provider-docker-native
+name: provider-dockerbuild
 runtime: dotnet
 plugins:
   providers:
-    - name: docker-native
+    - name: dockerbuild
       path: ../../bin