Set up Go modules.

This should make it easier for us to reuse Go code properly.
util/fipstools is kind of a mess. runner has been using relative
imports, but Go seems to prefer this mechanism these days.

Update-Note: The import spelling in ssl/test/runner changes. Also we now
    require Go 1.11. Or you could clone us into GOPATH, but no one does
    that.

Change-Id: I8bf91e1e0345b3d0b3d17f5c642fe78b415b7dde
Reviewed-on: https://boringssl-review.googlesource.com/31884
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/BUILDING.md b/BUILDING.md
index 9bf9cb2..ab9b71d 100644
--- a/BUILDING.md
+++ b/BUILDING.md
@@ -29,8 +29,9 @@
     and Clang should work on non-Windows platforms, and maybe on Windows too.
     To build the tests, you also need a C++ compiler with C++11 support.
 
-  * [Go](https://golang.org/dl/) is required. If not found by CMake, the go
-    executable may be configured explicitly by setting `GO_EXECUTABLE`.
+  * The most recent stable version of [Go](https://golang.org/dl/) is required.
+    If not found by CMake, the go executable may be configured explicitly by
+    setting `GO_EXECUTABLE`.
 
   * To build the x86 and x86\_64 assembly, your assembler must support AVX2
     instructions and MOVBE. If using GNU binutils, you must have 2.22 or later