Inspect post-evaluation exported variables to obtain rule names.

This is a much cleaner, more elegant approach than previous regex matching.
This still leaves room for unknown-name rule definitions, in case, for example, a user namespaces their rule definition not at the top level.
For example: "foo.bar = rule(...)"

RELNOTES: None.
PiperOrigin-RevId: 202380975

src/main/java/com/google/devtools/build/skydoc/rendering/BUILD

diff --git a/src/main/java/com/google/devtools/build/skydoc/rendering/BUILD b/src/main/java/com/google/devtools/build/skydoc/rendering/BUILD
index 13f2cc9..0dc5f08 100644
--- a/src/main/java/com/google/devtools/build/skydoc/rendering/BUILD
+++ b/src/main/java/com/google/devtools/build/skydoc/rendering/BUILD
@@ -15,6 +15,7 @@
     deps = [
         "//src/main/java/com/google/devtools/build/lib:events",
         "//src/main/java/com/google/devtools/build/lib:skylarkinterface",
+        "//src/main/java/com/google/devtools/build/lib:syntax",
         "//third_party:guava",
         "//third_party:jsr305",
     ],