ioutil.ReadAll didn't exist when we wrote that parser code
originally (in 2009). Now it does, so use it. This may also
make that code path slightly more efficient.

Also, now that we are guaranteed to have a fast path for reading
from an io.Reader (and thus an io.ReadCloser), simplify setup
code for parser.ParseFile calls in srcimporter.Importer.ParseFiles.

Remove the associated TODO since we cannot reproduce any significant
performance differences when running go test -run ImportStdLib for
the case where we used to read directly from a file (even before the
change to the parser).

Fixes #19281.

Change-Id: I816459d092bb9e27fdc85089b8f21d57ec3fd79a
Reviewed-on: https://go-review.googlesource.com/85395Reviewed-by: Alan Donovan <adonovan@google.com>