bp2build: codegen singleton lists on one line.

This CL refactors the bp2build code generator to pretty print lists with
a single element on one line, instead of taking up three lines, which
can make BUILD files unnecessarily long. A single line singleton list is
also more commonly used in BUILD files.

Test: TH
Change-Id: Ic9e44741bbb070c8f45925466b9ccdd0608498b2
diff --git a/bp2build/cc_library_headers_conversion_test.go b/bp2build/cc_library_headers_conversion_test.go
index 655218d..c59241f 100644
--- a/bp2build/cc_library_headers_conversion_test.go
+++ b/bp2build/cc_library_headers_conversion_test.go
@@ -141,30 +141,18 @@
         "dir-2/dir2a.h",
         "dir-2/dir2b.h",
     ] + select({
-        "//build/bazel/platforms/arch:arm64": [
-            "arch_arm64_exported_include_dir/a.h",
-        ],
-        "//build/bazel/platforms/arch:x86": [
-            "arch_x86_exported_include_dir/b.h",
-        ],
-        "//build/bazel/platforms/arch:x86_64": [
-            "arch_x86_64_exported_include_dir/c.h",
-        ],
+        "//build/bazel/platforms/arch:arm64": ["arch_arm64_exported_include_dir/a.h"],
+        "//build/bazel/platforms/arch:x86": ["arch_x86_exported_include_dir/b.h"],
+        "//build/bazel/platforms/arch:x86_64": ["arch_x86_64_exported_include_dir/c.h"],
         "//conditions:default": [],
     }),
     includes = [
         "dir-1",
         "dir-2",
     ] + select({
-        "//build/bazel/platforms/arch:arm64": [
-            "arch_arm64_exported_include_dir",
-        ],
-        "//build/bazel/platforms/arch:x86": [
-            "arch_x86_exported_include_dir",
-        ],
-        "//build/bazel/platforms/arch:x86_64": [
-            "arch_x86_64_exported_include_dir",
-        ],
+        "//build/bazel/platforms/arch:arm64": ["arch_arm64_exported_include_dir"],
+        "//build/bazel/platforms/arch:x86": ["arch_x86_exported_include_dir"],
+        "//build/bazel/platforms/arch:x86_64": ["arch_x86_64_exported_include_dir"],
         "//conditions:default": [],
     }),
 )`, `cc_library_headers(
@@ -173,18 +161,14 @@
         "lib-1/lib1a.h",
         "lib-1/lib1b.h",
     ],
-    includes = [
-        "lib-1",
-    ],
+    includes = ["lib-1"],
 )`, `cc_library_headers(
     name = "lib-2",
     hdrs = [
         "lib-2/lib2a.h",
         "lib-2/lib2b.h",
     ],
-    includes = [
-        "lib-2",
-    ],
+    includes = ["lib-2"],
 )`},
 		},
 		{
@@ -223,27 +207,13 @@
     name = "darwin-lib",
 )`, `cc_library_headers(
     name = "foo_headers",
-    deps = [
-        ":base-lib",
-    ] + select({
-        "//build/bazel/platforms/os:android": [
-            ":android-lib",
-        ],
-        "//build/bazel/platforms/os:darwin": [
-            ":darwin-lib",
-        ],
-        "//build/bazel/platforms/os:fuchsia": [
-            ":fuchsia-lib",
-        ],
-        "//build/bazel/platforms/os:linux": [
-            ":linux-lib",
-        ],
-        "//build/bazel/platforms/os:linux_bionic": [
-            ":linux_bionic-lib",
-        ],
-        "//build/bazel/platforms/os:windows": [
-            ":windows-lib",
-        ],
+    deps = [":base-lib"] + select({
+        "//build/bazel/platforms/os:android": [":android-lib"],
+        "//build/bazel/platforms/os:darwin": [":darwin-lib"],
+        "//build/bazel/platforms/os:fuchsia": [":fuchsia-lib"],
+        "//build/bazel/platforms/os:linux": [":linux-lib"],
+        "//build/bazel/platforms/os:linux_bionic": [":linux_bionic-lib"],
+        "//build/bazel/platforms/os:windows": [":windows-lib"],
         "//conditions:default": [],
     }),
 )`, `cc_library_headers(