build/fietsje: split into monogon-specific library and cli tool

This is a first pass at sightly modularizing fietsje. This allows
fietsje-for-Monogon to be used as a Go library, and moves all the
toolbase startup logic into its own executable package. This allows us
to call fietsje from some multi-purpose CI/check tool that I'm slowly
implementing on the side.

Fietsje should still be split up further, allowing a generic fietsje
library to be used for more than just the Monogon repository - but that
will come at a later point.

Change-Id: Ic59c0bb954c5416fda95d3604d5aa94553dc1030
Reviewed-on: https://review.monogon.dev/c/monogon/+/331
Reviewed-by: Mateusz Zalega <mateusz@monogon.tech>
diff --git a/build/fietsje/deps_containerd.go b/build/fietsje/deps_containerd.go
index 99d36f6..37c71c5 100644
--- a/build/fietsje/deps_containerd.go
+++ b/build/fietsje/deps_containerd.go
@@ -14,7 +14,9 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-package main
+package fietsje
+
+// deps_containerd.go contains all dependencies required by containerd/runc.
 
 func depsContainerd(p *planner) {
 	p.collectOverride(