Add path interposer

This will allow us to track (and eventually limit) the commands that the
build references via $PATH. These are mostly implicit dependencies on
the host system -- for Linux, we assume something similar to Ubuntu
14.04 with a few extra packages, but this will let us better define
that.

This will not catch uses of tools with absolute paths (/bin/bash, etc),
but most uses shouldn't be relying on absolute path names anyways.

Adds ~400ms on the first startup, ~140ms on subsequent runs, and
overhead of a few ms for every forwarded execution.

Test: m
Test: build/soong/build_test.bash
Test: Add `gcc --version`, TEMPORARY_DISABLE_PATH_RESTRICTIONS=true m
Change-Id: Id68cbb1c8ceef65bbbb10751e83722c7662d2351

ui/build/Android.bp

@@ -12,10 +12,23 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+bootstrap_go_package {
+    name: "soong-ui-build-paths",
+    pkgPath: "android/soong/ui/build/paths",
+    srcs: [
+        "paths/config.go",
+        "paths/logs.go",
+    ],
+    testSrcs: [
+        "paths/logs_test.go",
+    ],
+}
+
 bootstrap_go_package {
     name: "soong-ui-build",
     pkgPath: "android/soong/ui/build",
     deps: [
+        "soong-ui-build-paths",
         "soong-ui-logger",
         "soong-ui-tracer",
         "soong-shared",
@@ -33,6 +46,7 @@ bootstrap_go_package {
         "finder.go",
         "kati.go",
         "ninja.go",
+        "path.go",
         "proc_sync.go",
         "signal.go",
         "soong.go",