pkg/cover: fix function coverage in html reports

The HTML code assumes that files and functions match one-to-one
as they are identified by indices (file_N should match function_N).
Since we only add non-empty functions, this relation is broken
and a the report shows function coverage for a random file.
Moreover, since the order is based on map iteration (random each time),
function coverage shown for a file also randomly changes each time.

Follow up to #2074

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