deps(i18n): Move to external go-i18n and gotext

[didrocks]

We did the work to avoid our internal/ package last cycle. Move now to
it.
We still need to validate each lines that we log or not.

This removes the need for snapcore go-gettext.

UDENG-1990

[codecov-commenter]

Codecov Report

Attention: 60 lines in your changes are missing coverage. Please review.

Comparison is base (3c4da75) 86.00% compared to head (06b6526) 86.43%.

Files	Patch %	Lines
internal/watcher/watcher.go	75.00%	7 Missing ⚠️
internal/policies/apparmor/apparmor.go	81.25%	6 Missing ⚠️
internal/ad/admxgen/admxgen.go	73.68%	5 Missing ⚠️
internal/ad/admxgen/dconf/dconf.go	54.54%	5 Missing ⚠️
internal/watchdservice/watchdservice.go	83.87%	5 Missing ⚠️
internal/ad/ad.go	82.60%	4 Missing ⚠️
internal/adsysservice/doc.go	33.33%	4 Missing ⚠️
internal/authorizer/servercreds.go	40.00%	3 Missing ⚠️
internal/cmdhandler/suggest.go	40.00%	3 Missing ⚠️
internal/config/watchd/watchd.go	66.66%	3 Missing ⚠️
... and 10 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #886      +/-   ##
==========================================
+ Coverage   86.00%   86.43%   +0.42%     
==========================================
  Files          76       75       -1     
  Lines        8718     8675      -43     
==========================================
  Hits         7498     7498              
+ Misses        883      840      -43     
  Partials      337      337              

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[denisonbarbosa]

LGTM!