Move pkg into codecs/av1

[aler9]

Description

As previously discussed, the pkg directory is not used in Pion projects.

This maintains compatibility with existing code and adds a deprecation notice to the old objects.

Reference issue

Fixes #221

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (3fbe548) 87.38% compared to head (7d798b0) 87.38%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #237   +/-   ##
=======================================
  Coverage   87.38%   87.38%           
=======================================
  Files          20       20           
  Lines        1887     1887           
=======================================
  Hits         1649     1649           
  Misses        207      207           
  Partials       31       31           

Flag	Coverage Δ
go	87.38% <100.00%> (ø)
wasm	86.80% <95.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
codecs/av1_packet.go	96.59% <ø> (ø)
codecs/av1/frame/av1.go	100.00% <100.00%> (ø)
codecs/av1/obu/leb128.go	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.