improve report generation for multi-package reports

This pull-request improves the generated HTML for code coverage reports containing multiple packages.  It attempts to not change the output for single-package reports, but some changes did occur.

Previously multiple package reports resulted in an HTML file with multiple <html> sections inside it.  The "total" percentage shown in fixed position upper right corner would be misleading as it was the code coverage percentage of the last package, not a summary of the whole report.

This set of changes introduces a new package index at the top (only rendered if there is more than 1 package).  Further it abstracts the summary position shown in the upper-right-hand-corner so that a different value can be shown there in the case of multi-package reports.