Both recipes and packages have the `install.environment` property, so
refactor `runtime_environment()` out of the `Package` trait into its own
trait, then require both `Recipe` and `Package` implement it.

Signed-off-by: J Robert Ray <jrray@jrray.org>

In order to implement foreign traits for this type.

Signed-off-by: J Robert Ray <jrray@jrray.org>

Fixes #1081. The bug causing #1081 was in `InstallSpec::is_default` as the
logic was not updated when the `environment` field was added to the struct
way back at https://github.com/spkenv/spk/blob/f4899ce1e4ac0634ee95608e322a86a358096d4d/src/api/install_spec.rs#L31

As a guard against this kind of bug in the future, an `IsDefault` trait was
created and a derive macro created to derive the `is_default` method
semi-automatically. All existing `is_default` methods have been changed to
use the trait instead.

Signed-off-by: J Robert Ray <jrray@jrray.org>