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/build/sqlc/BUILD.bazel b/build/sqlc/BUILD.bazel
index 22fbb6a..60c24b3 100644
--- a/build/sqlc/BUILD.bazel
+++ b/build/sqlc/BUILD.bazel
@@ -9,4 +9,5 @@
         "netgo",
         "nowasm",
     ],
+    pure = "off",
 )