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.

testing/base/BUILD

@@ -82,8 +82,8 @@ cc_test(
         ":source_gen_lib",
         "//common:all_llvm_targets",
         "//common:set",
+        "//toolchain/base:install_paths_test_helpers",
         "//toolchain/driver",
-        "//toolchain/install:install_paths_test_helpers",
         "@googletest//:gtest",
         "@llvm-project//llvm:Support",
     ],

testing/base/source_gen_test.cpp

@@ -11,8 +11,8 @@
 
 #include "common/set.h"
 #include "testing/base/global_exe_path.h"
+#include "toolchain/base/install_paths_test_helpers.h"
 #include "toolchain/driver/driver.h"
-#include "toolchain/install/install_paths_test_helpers.h"
 
 namespace Carbon::Testing {
 namespace {