Move the :install_paths library to //toolchain/base (#6457)

Previously this was kept in `//toolchain/install` so it would be near to
the code that actually defines the installation layout. However, that
creates somewhat unfortunate dependency cycles between
`//toolchain/install` and other directories. Exacerbating this, a
subsequent PR is likely to add dependencies on it from
`//toolchain/base` itself that suggests that is the correct layering.

This PR just moves the code mechanically with as few other edits as
possible.

toolchain/base/install_paths_test_helpers.cpp

@@ -0,0 +1,33 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include "toolchain/base/install_paths_test_helpers.h"
+
+#include <memory>
+#include <utility>
+
+#include "testing/base/global_exe_path.h"
+
+namespace Carbon::Testing {
+
+// Prepares the VFS with prelude files from the real filesystem. Primarily for
+// tests.
+auto AddPreludeFilesToVfs(
+    const InstallPaths& install_paths,
+    llvm::IntrusiveRefCntPtr<llvm::vfs::InMemoryFileSystem>& vfs) -> void {
+  // Load the prelude into the test VFS.
+  auto real_fs = llvm::vfs::getRealFileSystem();
+  auto prelude = install_paths.ReadPreludeManifest();
+  CARBON_CHECK(prelude.ok(), "{0}", prelude.error());
+
+  for (const auto& path : *prelude) {
+    llvm::ErrorOr<std::unique_ptr<llvm::MemoryBuffer>> file =
+        real_fs->getBufferForFile(path);
+    CARBON_CHECK(file, "Error getting file: {0}", file.getError().message());
+    bool added = vfs->addFile(path, /*ModificationTime=*/0, std::move(*file));
+    CARBON_CHECK(added, "Duplicate file: {0}", path);
+  }
+}
+
+}  // namespace Carbon::Testing