make follow reexports more efficient (#7741)
### Description
When accessing many exports of a barrel file with many `export *` it was
a bit inefficient before since it was walking all star exports for every
accessed export.
After this change it only creates a map of the star exports once and
accessing every export only looks up the result in the map. This makes
it 4 times more efficient for a file with 140 `export *`. Probably even
more for bigger barrel files.
### Testing Instructions
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Closes PACK-2747
