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/test_binary.cpp

@@ -0,0 +1,5 @@
+// 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
+
+auto main() -> int { return 0; }