The initially empty ast.StdAst was created to break the circular
dependency. The generation of stdlib AST used to depend on
the primary "jsonnet" package, which meant that "jsonnet"
could not depend on it directly. Hence stdlib needed to be put
in the ast package. Now dumpstdlibast no longer depends on Jsonnet,
so we can get rid of this complication.
All code using ast.StdAst should now use astgen.StdAst.
Add go mod
Also make the build docs nicer and cleanup the file directory to be in line
with most other go projects. This also make it so you can build jsonnet
without setting -o on go build.
