feat: inline package declaration

Docsonnet packages previously used to be declared in a `main.docsonnet`.
Because this file basically always contains the same, it is kinda
redundant.

To avoid that, the information from there has been inlined into the main
Jsonnet file, while the parser logic will become part of the Go
application, as embedded Jsonnet.

doc-util/main.docsonnet

@@ -1,8 +1 @@
-local d = import './main.libsonnet';
-
-d.new('d', url='github.com/jsonnet-libs/docsonnet/doc-util')
-+ d.withAPI(import './main.libsonnet')
-+ d.withHelp(|||
-  `doc-util` provides a Jsonnet interface for `docsonnet`,
-   a Jsonnet API doc generator that uses structured data instead of comments.
-|||)
+(import "../load.libsonnet").package(import "./main.libsonnet")