split_on_line_directives: fix handling of single-file package case

split_on_line_directives breaks the input at #line directives and
returns a named list of lines for each file.

For a package with a single file under R/, there is one directive.
The bounds calculation is still correct for that case.  However, the
return value is incorrectly a matrix rather than a list because the
mapply call simplifies the result.

At this point, this bug is mostly [*] unexposed because this code path
is only triggered if utils::getParseData returns NULL, and it should
always return a non-NULL result for the single-file package case.  The
next commit will reorder things, exposing the bug.

Tell mapply to not simplify the result.

[*] The simplification to a matrix could also happen for multi-file
    packages in the unlikely event that all files have the same number
    of lines.

R/utils.R

@@ -96,7 +96,12 @@ split_on_line_directives <- function(lines) {
 
   file_starts <- directive_lines + 1
   file_ends <- c(directive_lines[-1] - 1, length(lines))
-  res <- mapply(function(start, end) lines[start:end], file_starts, file_ends)
+  res <- mapply(
+    function(start, end) lines[start:end],
+    file_starts,
+    file_ends,
+    SIMPLIFY = FALSE
+  )
   names(res) <- na.omit(matches$filename)
   res
 }