Refactor to support generic Python "zipapps"

[warsaw]

pex is just one implementation of a Python "zipapp". Users might want to support other types of zipapps, so this branch refactors pex support so as to add a generic ZipappExtension and related classes. Then it reimplements pex support on top of that.

This should not change existing pex support, for either fat or thin pexes. It also keeps the build.gradle settings for selecting thin vs fat pexes, but it also allows for a more generic specification. E.g.

python {
    pex {
        isFat = false
    }
}

(i.e. allows for isFat and fatPex for backward compatibility)

[warsaw]

Since this is a refactoring branch I didn't add any new tests, but the existing tests should still pass and provide a similar level of coverage.

[ghost]

I think this would should be an interface vs a class. We can make an abstract class to hold some helpers if needed.

[warsaw]

I made ZipappExtension a class to hold the common fields cache and isFat which are used in both the pex extension subclass, and the (currently only available in a separate non-public extension) shiv extension subclass. If we make ZipappExtension an interface, where will we put cache and isFat? Also, I think this class is more about providing common functionality for subclasses rather than defining a useful interface that other classes should implement. E.g., it's all about getting and setting the cache directory and the "fatness" of the artifacts.

[warsaw]

@Kopy07 Fair enough. I think now that we have the cache property sorted out, I agree with you about making ZipappExtension an interface. The next push will do that.

[ghost]

Is this class usable directly, or will people have to extend it?

[warsaw]

They will have to extend it to implement the buildEntryPoints() method. See ThinPexGenerator. This is essentially a refactor of the original ThinPexGenerator class for commonality with the internal thin shiv generator class.

[ghost]

Where is this captured now?

[warsaw]

The ZipappExtension class referenced above :)

[warsaw]

@Kopy07 The latest change turns ZipappExtension into an interface and adds some @Deprecated annotations as discussed off-line.