Make 1.x deprecation targets package-private

[travisbrown]

This PR makes everything that was deprecated in 1.x package-private (in the cases where that's possible without breaking bincompat).

This means less noise in the API docs, and that users will be in a better position to be ready once we can finally clean this stuff up for real.

I don't feel terribly strongly about getting this into 2.1.0, but I think it'd be nice to have.

[codecov-io]

Codecov Report

Merging #3154 into master will decrease coverage by 0.04%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #3154      +/-   ##
==========================================
- Coverage   93.15%   93.11%   -0.05%     
==========================================
  Files         376      376              
  Lines        7353     7353              
  Branches      204      190      -14     
==========================================
- Hits         6850     6847       -3     
- Misses        503      506       +3

Flag	Coverage Δ
#scala_version_212	93.45% <0%> (-0.05%)	⬇️
#scala_version_213	90.77% <0%> (-0.06%)	⬇️

Impacted Files	Coverage Δ
core/src/main/scala/cats/data/NonEmptyList.scala	98.7% <ø> (-0.65%)	⬇️
core/src/main/scala/cats/data/Kleisli.scala	90.9% <ø> (ø)	⬆️
core/src/main/scala/cats/package.scala	100% <ø> (ø)	⬆️
core/src/main/scala/cats/syntax/semigroupal.scala	50% <ø> (ø)	⬆️
free/src/main/scala/cats/free/Trampoline.scala	0% <ø> (ø)	⬆️
core/src/main/scala/cats/syntax/apply.scala	57.14% <ø> (ø)	⬆️
free/src/main/scala/cats/free/Coyoneda.scala	84.61% <ø> (ø)	⬆️
core/src/main/scala/cats/instances/duration.scala	66.66% <ø> (-33.34%)	⬇️
core/src/main/scala/cats/Apply.scala	78.94% <ø> (ø)	⬆️
...src/main/scala/cats/instances/finiteDuration.scala	0% <ø> (-100%)	⬇️
... and 4 more

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[rossabaker]

I'm often surprised from how far back people are upgrading, and usually err on the side of keeping deprecations around a long time, but these are implicits or pre-1.x. An out-of-character 👍 from me.

[travisbrown]

@rossabaker Out-of-character 👍s are the best 👍s. 😄