Add support for forcing a rebuild of runtimes (#6537)

This is useful during development, testing, and will also be useful for
a more bazel-integrated build step.

Also clean up the path management when creating runtimes:

- Teach the main runtimes code to handle making a relative path absolute
- Separate out methods for _creating_ a runtimes tree vs. opening an
existing one. Teach the creation path to create intervening directories
as needed. This provides a more useful and less surprising set of
behaviors.

Last but not least, also clean up a bunch of comments in the runtimes
cache code to talk generically about components -- these APIs are no
longer specific to the resource directory.

toolchain/driver/driver.cpp

@@ -206,9 +206,8 @@ auto Driver::RunCommand(llvm::ArrayRef<llvm::StringRef> args) -> DriverResult {
   driver_env_.runtimes_cache = std::move(*cache_result);
 
   if (!options.prebuilt_runtimes_path.empty()) {
-    auto result = Runtimes::Make(
-        std::filesystem::absolute(options.prebuilt_runtimes_path.str()),
-        driver_env_.vlog_stream);
+    auto result = Runtimes::OpenExisting(options.prebuilt_runtimes_path.str(),
+                                         driver_env_.vlog_stream);
     if (!result.ok()) {
       // TODO: We should provide a better diagnostic than the raw error.
       CARBON_DIAGNOSTIC(DriverPrebuiltRuntimesInvalid, Error, "{0}",