x/pkgsite: include needed init methods in playable examples

[jba]

An example may depend on an init function. When creating a playable example (standalone program) from an example, we currently do not include init functions, so such examples won't run correctly.

Example: https://pkg.go.dev/gonum.org/v1/gonum/graph/community#example-Profile-Multiplex

Challenges:

• We only want to include the init function if it affects the example, so we have to walk its body to check.
• A relevant init function can be in any file in the test package, and we only examine one file at a time at present.