Respect go_prefix attr of dependencies (#30)

Uses go_prefix attr of individual dependencies to calculate their
importpaths.  This allows users to use Bazel's external dependency
resolution -- {,new_}git_repository rules -- to manage Go libraries.

c.f. https://github.com/bazelbuild/rules_go/issues/16#issuecomment-216010843

WORKSPACE

diff --git a/WORKSPACE b/WORKSPACE
index 8dae17e..cb32097 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -2,3 +2,25 @@
 
 load("//go:def.bzl", "go_repositories")
 go_repositories()
+
+GLOG_BUILD = """
+load("@//go:def.bzl", "go_prefix", "go_library")
+
+go_prefix("github.com/golang/glog")
+
+go_library(
+    name = "go_default_library",
+    srcs = [
+        "glog.go",
+        "glog_file.go",
+    ],
+    visibility = ["//visibility:public"],
+)
+"""
+
+new_git_repository(
+    name = "com_github_golang_glog",
+    remote = "https://github.com/golang/glog.git",
+    commit = "23def4e6c14b4da8ac2ed8007337bc5eb5007998",
+    build_file_content = GLOG_BUILD,
+)