rename envbuilder.MagicDir to envbuilder.WorkingDir

[johnstcn]

          Not introduced by your PR but I want to point out that `MagicDir` is kinda unhelpful as a name; perhaps `EnvbuilderWorkingDir` would be more explicit?

Originally posted by @dannykopping in #183 (comment)

[leo-oliveira2]

Hello, can I do this?

[SasSwart]

I've submitted an initial PR to get us most of the way to closing this issue. I have a few questions that I would like to verify:

1. Do we include a CLI change to rename the MagicDirBase option to WorkingDirBase in the scope for this issue? (please verify @johnstcn). I'm happy to do so. I just want the scope verified.
2. I see the term magic used elsewhere in the code in relation to magic files, magic images and magic directives. Some of these come from the package I just renamed. Does magic have a specific definition that I'm unaware of? I fear that we might introduce confusion if the term magic somehow coupled these concepts.

[johnstcn]

1. Do we include a CLI change to rename the MagicDirBase option to WorkingDirBase in the scope for this issue? (please verify @johnstcn). I'm happy to do so. I just want the scope verified.

I would consider this in scope of this issue. It's not really a "CLI" change as such, as it's not an exposed CLI option. Only users who import coder/envbuilder would be affected by this.

2. I see the term magic used elsewhere in the code in relation to magic files, magic images and magic directives. Some of these come from the package I just renamed. Does magic have a specific definition that I'm unaware of? I fear that we might introduce confusion if the term magic somehow coupled these concepts.

It's just semantic bleed from MagicDir. If those can be renamed without any external effects, we could do so.