Bxl support for performing analysis on targets

Intro

As Bob and I continue to build out bxl we want users to be able to inspect the providers and actions for a given target label. In order to support this, we need to be able to provide access to AnalysisResult via starlark, obtained via a call to RuleAnalysisCalculation::get_analysis_result.

How to implement it?

Our three principle options are as follows:

1. BxlContext::analyze(targetlabel: ConfiguredTargetLabelLike), where ConfiguredTargetLabelLike accepts ConfiguredTargetLabel, ConfiguredTargetNode, or sets and lists of these things + acceptable strings.

In this scenario, we attach the analysis method onto the bxl context itself, and require that users pass in the target label-ish thing when they want to construct an analysis result. It's a little awkward in some ways because the analysis is more naturally a method on the argument being passed in and the BxlContext is a context that is needed to perform the calculation. On the other hand, this allows us to construct a type analogous to TargetExpr which can parse from a wide variety of different ConfiguredTarget like things (strings, nodes, labels, sets, ...). It also is a bit nice from an implementational standpoint since we don't have to pass the context around everywhere. This isn't a huge pro though, since we can stick it in the global eval field.

result = bxl.analyze(bxl.cquery.deps("foo"))

2. ConfiguredTargetLabel::analyze(), ConfiguredTargetNode::analyze(), ... where we carry around the BxlContext in the eval global field and implement analysis on each type that is target label like.

The pro of this one is that it's quite natural - you can take a ConfiguredStarlarkTargetLabel and then just ... call analyze() on it like you might expect to. The two downsides are that we have to propagate the context around behind the scenes, and we'll have to provide an implementation of analyze on everything that we'd like to have be able to be analyzable.

result = "root//bin:the_binary".analyze()
# but we don't support
"root//bin:the_binary".rdeps()


# instead this looks nice
nodes = ctx.cquery.deps("foo")
for n in nodes:
  # since we can now do
  nodes.label
  nodes.attrs.field

  # similarly access analysis
  nodes.analysis

3. BxlContext::analysis(): AnalysisContext where AnalysisContext exposes AnalysisContext::analyze(targetlabel: ConfiguredTargetLabelLike).

There's not really any pros of this approach except that it's similar to the flow for cquery where we return a cqueryctx object to call cquery methods through.

result = ctx.analysis().analyze("//lib:file1")

We can also restrict the API to require that users go through cquery to obtain a ConfiguredTargetNode prior to calling analysis, although we don't have to. I say that we don't have to because the get_analysis_result method mentioned above is configured to accept a label anyway.