Clean up and simplify existing mach bootstrap

- Default to interactive mode and remove the `--interactive` flag
- Use `--force` to skip interactivity
- Change MSVC dependency storage organization on disk: put each version
  into its own folder and directly refer to the versioned folders,
  providing immutability and making the installation list redundant
- Reuse `host_triple()` function to fix broken bootstrapper dispatching
- Simplify code:
  - Remove or inline many unused and redudant functions and variables
  - Prefer plain functions to classes
  - Consolidate into fewer files, remove unnecessary bootstrapper/ dir
- Improve Python style
- Sort dependency list

python/servo/packages.py

@@ -0,0 +1,26 @@
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this file,
+# You can obtain one at http://mozilla.org/MPL/2.0/.
+
+WINDOWS_GNU = set([
+    "diffutils",
+    "make",
+    "mingw-w64-x86_64-toolchain",
+    "mingw-w64-x86_64-freetype",
+    "mingw-w64-x86_64-icu",
+    "mingw-w64-x86_64-nspr",
+    "mingw-w64-x86_64-ca-certificates",
+    "mingw-w64-x86_64-expat",
+    "mingw-w64-x86_64-cmake",
+    "patch",
+    "patchutils",
+    "python2-setuptools",
+    "tar",
+])
+
+WINDOWS_MSVC = {
+    "cmake": "3.6.1",
+    "moztools": "0.0.1-5",
+    "ninja": "1.7.1",
+    "openssl": "1.0.1t-vs2015",
+}