This commit fixes two issues that prevent rollup-plugin-html from
being used with Rollup's watch mode:
1. Each run adds the entrypoint from the input HTML file to the
input modules. This commit clears the whole build state each run
so the inputs from earlier runs are not included in the later
builds.
2. Changes to the input HTML files are not detected. This is fixed
by adding Rollup's input file as a watch file if it is a valid
input for the plugin.