internal/proxy: port script from Go Cloud to create module proxy (#95)

I have created gs://wire-modules to store the Wire dependencies. I ran
the script in this commit to populate the contents of the GCS bucket and
am updating Travis to point to the new proxy.

Fixes #81

internal/proxy/README.md

@@ -0,0 +1,64 @@
+# Wire module proxy
+
+The Travis build for Wire uses a [Go module proxy][] for dependencies, for
+efficiency and reliability, and to enforce that all of our dependencies meet our
+license requirements.
+
+The proxy is set
+[here](https://github.com/google/wire/blob/master/.travis.yml#L48).
+
+[Go module proxy]: https://research.swtch.com/vgo-module
+
+## Updating the GCS bucket
+
+When you add a new dependency, the Travis build will fail. To add the new
+dependency to the proxy:
+
+1.  Ensure that the new dependency is covered under one of the `notice`,
+    `permissive`, or `unencumbered` licences under the categories defined by
+    [Google Open Source](https://opensource.google.com/docs/thirdparty/licenses/).
+2.  Gather the new set of dependencies and sync them to the GCS bucket.
+
+```bash
+# This approach was suggested by:
+# https://github.com/go-modules-by-example/index/tree/master/012_modvendor
+
+# Create a temporary directory where we'll create a module download cache.
+tgp="$(mktemp -d)"
+
+# Copy current module cache into temporary directory as basis.
+# Periodically, someone on the project should go through this process without
+# running this step to prune unused dependencies.
+mkdir -p "$tgp/pkg/mod/cache/download"
+gsutil -m rsync -r gs://wire-modules "$tgp/pkg/mod/cache/download"
+
+# Run this command in the master branch.
+# It runs "go mod download" in every module that we have in our repo,
+# filling the cache with all of the module dependencies we need.
+./internal/proxy/makeproxy.sh "$tgp"
+
+# Run the above command again in your branch that's adding a new dependency,
+# to ensure the cache has any new dependencies.
+
+# Move the temporary cache to modvendor/.
+rm -rf modvendor
+cp -rp "$tgp/pkg/mod/cache/download/" modvendor
+
+# Clean up the temporary cache.
+GOPATH="$tgp" go clean -modcache
+rm -rf "$tgp"
+unset tgp
+
+# Synchronize modvendor to the proxy.
+
+# -n: preview only
+# -r: recurse directories
+# -c: compare checksums not write times
+# -d: delete remote files that aren't present locally
+gsutil rsync -n -r -c -d modvendor gs://wire-modules
+# If the set of packages being added and removed looks good,
+# repeat without the -n.
+
+# Once you are done...
+rm -rf modvendor
+```