depot_tools: add infra_paths recipe module for infra-specific paths

recipe_modules/git/example.expected/basic.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_MODULE[depot_tools::git]/resources/git_setup.py",
       "--path",
-      "[SLAVE_BUILD]/src",
+      "[CWD]/src",
       "--url",
       "https://chromium.googlesource.com/chromium/src.git"
     ],
-    "cwd": "[SLAVE_BUILD]",
     "name": "git setup"
   },
   {
     "cmd": [
       "git",
       "retry",
       "fetch",
       "origin",
       "master",
       "--recurse-submodules"
     ],
-    "cwd": "[SLAVE_BUILD]/src",
+    "cwd": "[CWD]/src",
     "name": "git fetch"
   },
   {
     "cmd": [
       "git",
       "checkout",
       "-f",
       "FETCH_HEAD"
     ],
-    "cwd": "[SLAVE_BUILD]/src",
+    "cwd": "[CWD]/src",
     "name": "git checkout"
   },
   {
     "cmd": [
       "git",
       "rev-parse",
       "HEAD"
     ],
-    "cwd": "[SLAVE_BUILD]/src",
+    "cwd": "[CWD]/src",
     "name": "read revision",
     "stdout": "/path/to/tmp/",
     "~followup_annotations": [
       "@@@STEP_TEXT@<br/>checked out 'deadbeef'<br/>@@@"
     ]
   },
   {
     "cmd": [
       "git",
       "clean",
       "-f",
       "-d",
       "-x"
     ],
-    "cwd": "[SLAVE_BUILD]/src",
+    "cwd": "[CWD]/src",
     "name": "git clean"
   },
   {
     "cmd": [
       "git",
       "submodule",
       "sync"
     ],
-    "cwd": "[SLAVE_BUILD]/src",
+    "cwd": "[CWD]/src",
     "name": "submodule sync"
   },
   {
     "cmd": [
       "git",
       "submodule",
       "update",
       "--init",
       "--recursive"
     ],
-    "cwd": "[SLAVE_BUILD]/src",
+    "cwd": "[CWD]/src",
     "name": "submodule update"
   },
   {
     "cmd": [
       "git",
       "-c",
       "foo=bar",
       "count-objects",
       "-v"
     ],
-    "cwd": "[SLAVE_BUILD]/src",
+    "cwd": "[CWD]/src",
     "name": "count-objects",
     "stdout": "/path/to/tmp/"
   },
   {
     "cmd": [
       "git",
       "config",
       "--get",
       "remote.origin.url"
     ],
-    "cwd": "[SLAVE_BUILD]/src",
+    "cwd": "[CWD]/src",
     "name": "git config remote.origin.url",
     "stdout": "/path/to/tmp/",
     "~followup_annotations": [
       "@@@STEP_TEXT@foo@@@"
     ]
   },
   {
     "cmd": [
       "git",
       "show",
       "HEAD",
       "--format=%at",
       "-s"
     ],
-    "cwd": "[SLAVE_BUILD]/src",
+    "cwd": "[CWD]/src",
     "name": "git show",
     "stdout": "/path/to/tmp/"
   },
   {
     "cmd": [
       "git",
       "fetch",
       "origin",
       "--tags"
     ],
-    "cwd": "[SLAVE_BUILD]/src",
+    "cwd": "[CWD]/src",
     "name": "git fetch tags"
   },
   {
     "cmd": [
       "git",
       "status"
     ],
-    "cwd": "[SLAVE_BUILD]/src",
+    "cwd": "[CWD]/src",
     "name": "git status"
   },
   {
     "cmd": [
       "git",
       "status"
     ],
-    "cwd": "[SLAVE_BUILD]/src",
+    "cwd": "[CWD]/src",
     "name": "git status can_fail_build"
   },
   {
     "cmd": [
       "git",
       "status"
     ],
-    "cwd": "[SLAVE_BUILD]/src",
+    "cwd": "[CWD]/src",
     "name": "git status cannot_fail_build"
   },
   {
     "cmd": [
       "git",
       "rebase",
       "origin/master"
     ],
-    "cwd": "[SLAVE_BUILD]/src",
+    "cwd": "[CWD]/src",
     "name": "my repo rebase"
   },
   {
     "cmd": [
       "git",
       "bundle",
       "create",
-      "[SLAVE_BUILD]/all.bundle",
+      "[CWD]/all.bundle",
       "--all"
     ],
-    "cwd": "[SLAVE_BUILD]/src",
+    "cwd": "[CWD]/src",
     "name": "git bundle"
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]