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
(cherry picked from https://android-review.googlesource.com/q/commit:3ba92d8f8083cde35e24b827e8dd198d63e5ec21)
Merged-In: I4a842e0d2055c4c5e42072b133f0afc3765c5b5c
Change-Id: I4a842e0d2055c4c5e42072b133f0afc3765c5b5c