Never keep sources in `parse_*()`

[DavisVaughan]

Closes #1603

It turns out that in both rlang 1.0.6 and 1.1.0, source information was already being dropped in all cases (I wanted to ensure that removing usage of flatten() was not the result of this change, and it doesn't seem to be).

I think this makes sense as the source reference information is typically attached to the expression object, and:

• parse_expr() returns the individual expression element, not the expression vector
• parse_exprs() can parse and flatten multiple expressions, and since each expression can return >1 element from parse(), you don't have a good 1:1 mapping between the source information and the flattened parsed expressions, so it makes sense to just drop the source information here too

[DavisVaughan]

I suppose you can manually merge the srcref list, but I'm not sure what you'd do with $wholeSrcref here

attributes(parse(text = "foo(); bar"))
#> $srcref
#> $srcref[[1]]
#> foo()
#> 
#> $srcref[[2]]
#> bar
#> 
#> 
#> $srcfile
#> <text> 
#> 
#> $wholeSrcref
#> foo(); bar

attributes(parse(text = "baz"))
#> $srcref
#> $srcref[[1]]
#> baz
#> 
#> 
#> $srcfile
#> <text> 
#> 
#> $wholeSrcref
#> baz

[lionel-]

hmm I think this attribute is not required.