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 b/BUILD
index 2b9c78c..7234a41 100644
--- a/BUILD
+++ b/BUILD
@@ -8,7 +8,7 @@
 
 alias(
     name = "fietsje",
-    actual = "//build/fietsje",
+    actual = "//build/fietsje/cmd",
 )
 
 # Shortcut for the Go SDK