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/planner.go b/build/fietsje/planner.go
index be955cf..0be1b8a 100644
--- a/build/fietsje/planner.go
+++ b/build/fietsje/planner.go
@@ -14,7 +14,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-package main
+package fietsje
 
 import (
 	"fmt"
@@ -25,7 +25,7 @@
 
 // planner is a builder for a single world of Go package dependencies, and what is
 // then emitted into a Starlark file containing gazelle go_repository rules. The
-// planner's builder system covers three increasingly specific contextx:
+// planner's builder system covers three increasingly specific contexts:
 //  - planner (this structure, allows for 'collecting' in high-level dependencies. ie. collections)
 //  - collection (represents what has been pulled in by a high-level dependency, and allows for 'using' transitive
 //    dependencies from a collection)