declarative script plugin dependencies

It is common that script plugins will need to be applied in a specific order. It would be nice if these scripts gave nice declarative error messages if this was violated. Here is a draft API:

```java
/**
* Throws an error if `('base/java')` hasn't already been applied.
* (Must be called only from within a script plugin.)
*/
void 干.requires('base/java') { ... }

/**
* Throws an error if `('base/scala')` has been applied,
* or if it is applied in the future.
* (Must be called only from within a script plugin.)
*/
void 干.mutuallyExclusiveWith('base/scala') { ... }

/**
* If `base/java` has already been applied, or is never applied, then this is fine.
* But if `base/java` is ever applied in the future, then there will be an error,
* because this plugin must run after 'base/java'.
* (Must be called only from within a script plugin.)
*/
void 干.mustRunAfter('base/java') { ... }

void 干.mustRunBefore('base/java') { ... }
```