Add FileSpec output path

[scottjasso]

Is your feature request related to a problem? Please describe.
I'm writing a gradle plugin to generate code from some source files, using gradle's InputChanges API.

• When a source file is added, I create the FileSpec for that file and call writeTo(outputDirectory).
• When a source file is removed, I want to delete the corresponding generated file. However, I have to calculate the generated file's relative path myself, because writeTo hides the details of that.

Describe the solution you'd like
An API like FileSpec.relativePath that gives the same relative path that writeTo(File) adds to the base directory.
Then I can call something like outputDirectory.resolve(fileSpec.relativePath).delete()

Describe alternatives you've considered
I can write my own FileSpec extension.

Additional context
The details aren't too complex to copy myself, I'm just worried that it could potentially get out of sync.

[scottjasso]

@JakeWharton I don't think that works. That gives the path after writing. I want to know the path without calling writeTo().

[JakeWharton]

Yes I misread the issue. Seems reasonable.

[scottjasso]

Thank you!