Multiple grammars not always recompiled

[jayhale]

Describe the bug
When using multiple grammars as described in #758:

use pest::Parser;
use pest_derive::Parser;

#[derive(Parser)]
#[grammar = "base.pest"]
#[grammar = "core.pest"]
struct SplitGrammarParser;

The parser is only recompiled with new grammars from both files when the last grammar is changed. I.e., changes to only base.pest aren't captured in debug builds until something in core.pest changes.

To Reproduce
Steps to reproduce the behavior:

• Create a simple grammar split across two files
• Use derive as above
• Experiment with changes in either or both grammar files (e.g., with cargo watch -x test)

Expected behavior

A change to any grammar should cause the parser to recompile with updates from all grammar files.

[NoahTheDuke]

Thanks for the report!

[tomtau]

CC @huacnlee