Switch to the other repository name. (#4024)

Despite this name not being in the runfiles manifest, this works in
Bazel because of how the module and workspace are configured. I should
have realized that as it was used elsewhere as well. This should
consolidate all of our runfiles paths to use the exact same patterns
now. These were the only remaining divergences.

toolchain/install/install_paths.cpp

@@ -16,7 +16,7 @@ namespace Carbon {
 
 // The location within our Bazel output tree of the prefix_root.
 static constexpr llvm::StringLiteral PrefixRoot =
-    "_main/toolchain/install/prefix_root/";
+    "carbon/toolchain/install/prefix_root/";
 
 // Path within an install prefix for our marker of a valid install.
 static constexpr llvm::StringLiteral MarkerPath =

toolchain/install/install_paths_test.cpp

@@ -81,7 +81,7 @@ class InstallPathsTest : public ::testing::Test {
 
 TEST_F(InstallPathsTest, PrefixRootDriver) {
   std::string installed_driver_path = test_runfiles_->Rlocation(
-      "_main/toolchain/install/prefix_root/bin/carbon");
+      "carbon/toolchain/install/prefix_root/bin/carbon");
 
   auto paths = InstallPaths::MakeExeRelative(installed_driver_path);
   ASSERT_THAT(paths.error(), Eq(std::nullopt)) << *paths.error();
@@ -90,7 +90,7 @@ TEST_F(InstallPathsTest, PrefixRootDriver) {
 
 TEST_F(InstallPathsTest, PrefixRootExplicit) {
   std::string marker_path = test_runfiles_->Rlocation(
-      "_main/toolchain/install/prefix_root/lib/carbon/carbon_install.txt");
+      "carbon/toolchain/install/prefix_root/lib/carbon/carbon_install.txt");
 
   llvm::StringRef prefix_path = marker_path;
   CARBON_CHECK(prefix_path.consume_back("lib/carbon/carbon_install.txt"))
@@ -109,7 +109,7 @@ TEST_F(InstallPathsTest, TestRunfiles) {
 
 TEST_F(InstallPathsTest, BinaryRunfiles) {
   std::string test_binary_path =
-      test_runfiles_->Rlocation("_main/toolchain/install/test_binary");
+      test_runfiles_->Rlocation("carbon/toolchain/install/test_binary");
   CARBON_CHECK(llvm::sys::fs::can_execute(test_binary_path))
       << test_binary_path;