ctexplain: first functional check-in

[gregestren]

#11511 set up basic project structure. This PR adds minimum working functionality.

Specifically, you can run it with a build command and it reports basic stats on the build's graph.

Example:

$ bazel-bin/tools/ctexplain/ctexplain -b "//testapp:foo"
Collecting configured targets for //testapp:foo... done in 0.62 s.

Configurations: 3
Targets: 79
Configured targets: 92 (+16.5% vs. targets)
Targets with multiple configs: 13

Notes:

• Changed import structure to prefer module imports over function, class imports (style guide recommendation)
• Set up structure for injecting arbitrary analyses. Each analysis consumes the build's set of configured targets and can output whatever it wants.
• Implemented one basic analysis
• Structured code to make it easy to fork output formatters (e.g. for machine-readable output). But tried not to add speculative inheritance / boilerplate too soon

Context: Measuring Configuration Overhead.
Work towards #10613

[meisterT]

cc @kkress

[sdtwigg]

The implementation and approach is fine. Mostly stylistic concerns.

[sdtwigg]

Pytype should let you annotate the return type. This ends up making most of the docstring redundant.

def analyze(cts: Tuple[ConfiguredTarget, ...]) -> _Summary:
  """Runs the analysis on a build's configured targets."""

[gregestren]

Done.

I remain confused about when pytype insists on an Args description and when not. Do you know?

[sdtwigg]

Assuming you mean pylint but, in my experience, it requires it when the docstring is >1 line.

There was talk about relaxing the requirement when pytype is used; however, do not know if that was ever actually executed.

[sdtwigg]

Error if unexpected arguments remain? e.g.

      if len(argv) > 1:
        raise app.UsageError('Too many command-line arguments.')

[gregestren]

FLAGS already catches unknown arguments. Would this be redundant?

[sdtwigg]

Without proof, I think this catches adding arguments after just --. I would need to experiment since nothing showed up on a cursory glance.