Remove unneeded packages from stdlib (#4093)

**What type of PR is this?**
perf improvement

**What does this PR do? Why is it needed?**
This cuts down the time to build the stdlib from 8.1 to 7.4s in my
testing and it also reduces the size of the `sdk.libs` depset and the
resulting linkcfg files.

I wasn't sure if it's ok to exclude all of `cmd/`; I suspect it should
be ok since the toolchain has prebuilt tools anyway, in which case we
can simplify the glob pattern.

**Which issues(s) does this PR fix?**

Fixes #

**Other notes for review**

go/private/BUILD.sdk.bazel

@@ -23,7 +23,21 @@ filegroup(
 
 filegroup(
     name = "srcs",
-    srcs = glob(["src/**"]),
+    srcs = glob(
+        ["src/**/*"],
+        exclude = [
+            "src/**/*_test.go",
+            "src/**/testdata/**",
+            "src/cmd/**",
+            "src/log/slog/internal/benchmarks/**",
+            "src/log/slog/internal/slogtest/**",
+            "src/internal/obscuretestdata/**",
+            "src/internal/testpty/**",
+            "src/net/internal/socktest/**",
+            "src/reflect/internal/example*/**",
+            "src/runtime/internal/startlinetest/**",
+        ],
+    ),
 )
 
 filegroup(