pdl: rename “grammar” to “file” throughout

The word “grammar” describes the rules which determine how to build
the AST. So the top-level AST node is not a “grammar”. Here I chose to
call it a “file”.

Bug: 239150180
Test: m pdl
Change-Id: I4a842e0d2055c4c5e42072b133f0afc3765c5b5c