build/toolbase: init

In an effort to better the developer/CI experience, I'm moving some of
our presubmit checks into a Go tool. This is a helper library that will
be used to interact with a Monogon workspace checkout from Go, both in
the new presubmit tool but also any other future tools that would like
to operate on source code.

Change-Id: Ie5f1b1d0153a1c853c241e167d2d3a469c636c94
Reviewed-on: https://review.monogon.dev/c/monogon/+/328
Reviewed-by: Mateusz Zalega <mateusz@monogon.tech>
diff --git a/build/toolbase/doc.go b/build/toolbase/doc.go
new file mode 100644
index 0000000..8bf9e25
--- /dev/null
+++ b/build/toolbase/doc.go
@@ -0,0 +1,3 @@
+// toolbase implements common functionality for tools operating on a Monogon
+// workspace, notably CI and developer tools.
+package toolbase