avoid exposing private type in API

[bbjubjub2494]

Fix #209. breaks binary compatibility.

[bbjubjub2494]

Maybe a [use case] simplified signature could be used between now and the next binary-incompatible release?

[typesafe-tools]

The validator has checked the following projects, tested using dbuild, projects built on top of each other.

Project	Reference	Commit
sbt	develop	sbt/sbt@4c55383
zinc	develop	sbt/zinc@9bef954
io	pull/210/head	6a1a47c
librarymanagement	develop	sbt/librarymanagement@9107125
util	develop	sbt/util@2e0d2d9

❌ The result is: FAILED
(restart)

[eed3si9n]

In https://jenkins.scala-sbt.org:8592/job/sbt-validator/1259/consoleFull, Dbuild is reporting that this will cause

[sbt212:error] java.lang.StackOverflowError
[sbt212:error] 	at sbt.internal.util.Init$Value.$anonfun$apply$3(Settings.scala:814)
[sbt212:error] 	at sbt.internal.util.Init$Value.$anonfun$apply$3(Settings.scala:814)
...

during

[sbt212] [error] Error during tests:
[sbt212] [error] 	sbt.internal.util.SettingsTest

I am kind of confused why this is the case.

[typesafe-tools]

A validation involving this pull request is in progress...

[typesafe-tools]

The validator has checked the following projects, tested using sbt-allsources.sh.

Project	Reference	Commit
sbt	develop	sbt/sbt@4c55383
zinc	develop	sbt/zinc@9bef954
io	pull/210/head	6a1a47c
librarymanagement	develop	sbt/librarymanagement@9107125
util	develop	sbt/util@2e0d2d9

❌ The result is: FAILED
(restart)

[typesafe-tools]

A validation involving this pull request is in progress...

[typesafe-tools]

The validator has checked the following projects, tested using sbt-allsources.sh.

Project	Reference	Commit
sbt	develop	sbt/sbt@4c55383
zinc	develop	sbt/zinc@9bef954
io	pull/210/head~	ca872a2
librarymanagement	develop	sbt/librarymanagement@9107125
util	develop	sbt/util@2e0d2d9

❌ The result is: FAILED
(restart)

[bbjubjub2494]

Rebased onto develop, I seemed to have missed some recent commits for unknown reasons.

[typesafe-tools]

A validation involving this pull request is in progress...

[typesafe-tools]

The validator has checked the following projects, tested using dbuild, projects built on top of each other.

Project	Reference	Commit
sbt	develop	sbt/sbt@4c55383
zinc	develop	sbt/zinc@9bef954
io	pull/210/merge
librarymanagement	develop	sbt/librarymanagement@9107125
util	develop	sbt/util@2e0d2d9

❌ The result is: FAILED
(restart)

[typesafe-tools]

A validation involving this pull request is in progress...

[typesafe-tools]

The validator has checked the following projects, tested using dbuild, projects built on top of each other.

Project	Reference	Commit
sbt	develop	sbt/sbt@4c55383
zinc	develop	sbt/zinc@9bef954
io	pull/210/head	c4f6dc7
librarymanagement	develop	sbt/librarymanagement@9107125
util	develop	sbt/util@2e0d2d9

❌ The result is: FAILED
(restart)

[bbjubjub2494]

Well it seems like I learned the hard way that @usecase is a terrible idea and already deprecated. (See scala/scala-dev#584) It chokes on vals and on defaulted parameters, so I can't make it work on all the definitions I would like to. Let's not do that.

As it stands, this PR targets 2.0.

[eed3si9n]

I'll create a branch to track this for future merge.