Disallow write-only task properties

[l0rinc]

E.g.

// no field, just setter
public void setMutablePropertyWithoutGetter() {} 

(see ValidateTaskPropertiesIntegrationTest#report setters for property with mutable type #type)

It should warn about a write-only property, i.e. unpaired get/set.
See: https://github.com/gradle/gradle/pull/9324/files#r280388131

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[lptr]

I think we want to do this eventually.

[lptr]

Another good validation to add for 7.0.

[lptr]

I tried to implement this, but it doesn't make much sense in the end. We also have tasks that have setters without getters, and in general there should be nothing wrong with having differently setters for properties if the different names add useful context.