This also replaces package_resolver with package_config, since
package_resolver is archived and is incompatible with null-safe Dart
packages.

This allows us to be statically explicit about when the expression
does or doesn't exist.

This is more friendly to null-safe code, since we no longer rely on
`_inStyleRule` implying that `_styleRule != null`.

This doesn't explicitly help null-safety, but it makes the
relationship between `Declaration.value` and `Declaration.children`
more obvious.

It was always valid to pass in `null`, this just makes it more clear.

This gets the project analyzer-clean, but the tests are still failing.

"Extender" is also commonly used to refer to the parent selector of an
`@extend` rule, so this helps disambiguate.

This gets rid of the weird subset of "one-off" extensions which didn't
have target information available. Now instead, each method explicitly
declares whether it takes/returns extensions (which do have target
info) or extenders (which do not).