treewide: enable pure Go by default

Pure can be safely enabled by default. For those targets which need cgo,
we can set `pure = "off"` on the go_binary or go_test.

This simplifies transitions, and makes the files/files_cc distinction
obsolete.

Change-Id: Ic1a985e7d347a7222f55735c1ee2016e8a7e1c65
Reviewed-on: https://review.monogon.dev/c/monogon/+/4158
Tested-by: Jenkins CI
Reviewed-by: Tim Windelschmidt <tim@monogon.tech>
diff --git a/osbase/bootparam/BUILD.bazel b/osbase/bootparam/BUILD.bazel
index 5a5704e..b688845 100644
--- a/osbase/bootparam/BUILD.bazel
+++ b/osbase/bootparam/BUILD.bazel
@@ -18,6 +18,7 @@
     ],
     embed = [":bootparam"],
     gc_goopts = ["-d=libfuzzer"],
+    pure = "off",
     deps = [
         "//osbase/bootparam/ref",
         "@com_github_google_go_cmp//cmp",