add type-checked get_field() method to TargetAdaptor and consume v1 target payloads

[cosmicexplorer]

Problem

One possible attempt to address the migration to v2 TargetAdaptors, in conjunction with #9153. The get_field() method makes progress towards #4535.

This implements the strategy from #8760, much more cleanly and efficiently.

Solution

• For several TargetAdaptor subclasses (such as PythonBinaryAdaptor), assign a v1 Target subclass (such as PythonBinary) to the _v1_target_class class field in engine_initializer.py.
• When such a TargetAdaptor is accessed, create an instance of self._v1_target_class with build_graph=None and sources=None.
• Add all payload fields from the constructed v1 Target to the TargetAdaptor's kwargs().
• Add the get_field() method to TargetAdaptor to access its kwargs while checking types.

Result

If a v2 TargetAdaptor has declared a _v1_target_class field, it will have access to that target's payload fields when created. This allows re-use of logic written in v1 target classes, while allowing a smooth eventual migration to v2 TargetAdaptors. Along with #9153, this is a first step in addressing #4535.

[Eric-Arellano]

Closing due to being superseded by the Target API.