[flutter_plugin_tools] Reimplements the excerpt system inline in the …

…tool, rather than relying on a separate package. (#4417)

* Allows excerpts to come from any package, not just examples.
* Fixes a bug in the excerpting logic that was causing a stray `}` to appear in one example.
* Removes the need for `build.excerpt.yaml` files.
* Remove the dependency on build_runner for excerpts.
* Reduces the time to generate the excerpts from about 10 minutes to about 5 seconds.
* Almost certainly fixes flutter/flutter#107180 (untested).

The new logic is not quite backwards compatible; the `path-base` feature now specifies a real path to the actual source directories, rather than a path into the invisible generated `excerpts/` directory with its special structure. Also, a number of features from the previous package that were not actually used in this repository are no longer supported (such as having multiple section names per `#docregion` pragma).

README.md

@@ -1,4 +1,4 @@
-<?code-excerpt path-base="excerpts/packages/video_player_example"?>
+<?code-excerpt path-base="example/lib"?>
 
 # Video Player plugin for Flutter
 

test/web_vtt_test.dart

@@ -154,7 +154,7 @@ WEBVTT
 00:05.200 --> 00:06.000 align:start size:50%
 <v Roger Bingham><i>You know I'm so excited my glasses are falling off here.</i>
 
-00:00:06.050 --> 00:00:06.150 
+00:00:06.050 --> 00:00:06.150
 <v Roger Bingham><i>I have a different time!</i>
 
 00:06.200 --> 00:06.900