Merge InputAccessor into SourceAccessor

After the removal of the InputAccessor::fetchToStore() method, the
only remaining functionality in InputAccessor was `fingerprint` and
`getLastModified()`, and there is no reason to keep those in a
separate class.

src/libexpr/parser-state.hh

@@ -44,7 +44,7 @@ struct ParserState
     Expr * result;
     SourcePath basePath;
     PosTable::Origin origin;
-    const ref<InputAccessor> rootFS;
+    const ref<SourceAccessor> rootFS;
     const Expr::AstSymbols & s;
 
     void dupAttr(const AttrPath & attrPath, const PosIdx pos, const PosIdx prevPos);