[sdk] Added dist_dir property to the sysroot SDK atom.

Bug: DX-538

Testing:
* sdk archive builds correctly.
* workspace generated properly
* tests pass as long as I use the same version of bazel that infra uses (0.17.2)

Change-Id: I808d1e6ca9d66b56ba6ad8f2e3a06270fd49c79d

build/sdk/meta/sysroot.json

@@ -69,6 +69,10 @@
             "$ref": "common.json#/definitions/file"
           }
         },
+        "dist_dir": {
+          "description": "Path to the base directory for prebuilt libraries",
+          "$ref": "common.json#/definitions/file"
+        },
         "include_dir": {
           "description": "Path to the base directory for includes",
           "$ref": "common.json#/definitions/file"
@@ -101,7 +105,8 @@
         "include_dir",
         "link_libs",
         "dist_libs",
-        "debug_libs"
+        "debug_libs",
+        "dist_dir"
       ],
       "additionalProperties": false
     }

zircon/public/sysroot/BUILD.gn

@@ -17,6 +17,7 @@ version_content = {
   root = file_base
 
   include_dir = "$file_base/include"
+  dist_dir = ""
   headers = []
   link_libs = []
   dist_libs = []
@@ -75,6 +76,7 @@ foreach(entry, zircon_legacy_sysroot) {
       } else if (defined(sdk.dist)) {
         file = "$sysroot_dir/${sdk.dist}"
         version_content.dist_libs += [ "$file_base/${sdk.dist}" ]
+        version_content.dist_dir = file_base
         sdk_files += [
           {
             source = rebase_path(sdk.source, "", zircon_root_build_dir)