Update all submodules that rustbuild doesn't depend on lazily

This only updates the submodules the first time they're needed, instead
of unconditionally the first time you run x.py.

Ideally, this would move *all* submodules and not exclude some tools and
backtrace. Unfortunately, cargo requires all `Cargo.toml` files in the
whole workspace to be present to build any crate.

On my machine, this takes the time for an initial submodule clone (for
`x.py --help`) from 55.70 to 15.87 seconds.

This uses exactly the same logic as the LLVM update used, modulo some
minor cleanups:
- Use a local variable for `src.join(relative_path)`
- Remove unnecessary arrays for `book!` macro and make the macro simpler to use
- Add more comments

src/bootstrap/bootstrap.py

@@ -989,21 +989,30 @@ class RustBuild(object):
         slow_submodules = self.get_toml('fast-submodules') == "false"
         start_time = time()
         if slow_submodules:
-            print('Unconditionally updating all submodules')
+            print('Unconditionally updating submodules')
         else:
             print('Updating only changed submodules')
         default_encoding = sys.getdefaultencoding()
-        submodules = [s.split(' ', 1)[1] for s in subprocess.check_output(
-            ["git", "config", "--file",
-             os.path.join(self.rust_root, ".gitmodules"),
-             "--get-regexp", "path"]
-        ).decode(default_encoding).splitlines()]
+        # Only update submodules that are needed to build bootstrap.  These are needed because Cargo
+        # currently requires everything in a workspace to be "locally present" when starting a
+        # build, and will give a hard error if any Cargo.toml files are missing.
+        # FIXME: Is there a way to avoid cloning these eagerly? Bootstrap itself doesn't need to
+        #   share a workspace with any tools - maybe it could be excluded from the workspace?
+        #   That will still require cloning the submodules the second you check the standard
+        #   library, though...
+        # FIXME: Is there a way to avoid hard-coding the submodules required?
+        # WARNING: keep this in sync with the submodules hard-coded in bootstrap/lib.rs
+        submodules = [
+            "src/tools/rust-installer",
+            "src/tools/cargo",
+            "src/tools/rls",
+            "src/tools/miri",
+            "library/backtrace",
+            "library/stdarch"
+        ]
         filtered_submodules = []
         submodules_names = []
         for module in submodules:
-            # This is handled by native::Llvm in rustbuild, not here
-            if module.endswith("llvm-project"):
-                continue
             check = self.check_submodule(module, slow_submodules)
             filtered_submodules.append((module, check))
             submodules_names.append(module)