The `Executable` class emits a warning when you pass quoted arguments to it:
```
> spack pkg grep '"namespace"'
==> Warning: Quotes in command arguments can confuse scripts like configure.
The following arguments may cause problems when executed:
"namespace"
Quotes aren't needed because spack doesn't use a shell. Consider removing them.
If multiple levels of quotation are required, use `ignore_quotes=True`.
```
This is to warn new package authors who aren't used to calling build commands in python.
It's not useful for `spack pkg grep`, where we really are passing args on the command
line, and if we pass a quoted string, we probably meant to.
- [x] make `ignore_quotes` an instance variable, not just an argument to ``__call__`
- [x] set `ignore_quotes` to `True` in `spack pkg grep`
Signed-off-by: Todd Gamblin <tgamblin@llnl.gov>
a841ddd00c