Hotfix for 0.6.4 breaking plugins, along with 2 other compiler bugfixes.

#### Bugfixes ⛑️

- Fix executable plugins that implement standalone router [#1910](https://github.com/terrastruct/d2/pull/1910)
- Fix compiler error with multiple nested spread substitutions [#1913](https://github.com/terrastruct/d2/pull/1913)
- Fix substitutions from imports into different scopes [#1914](https://github.com/terrastruct/d2/pull/1914)