Stop using os-lib and upickle(?)

[keynmol]

Unfortunately, it's a bit of a minefield when used with Ammonite, which brings in its own versions.

I had to to play a bit of Jenga with versions and still get a bincompat error during building a blog:

java.lang.NoSuchMethodError: 'boolean os.copy$.apply$default$7()'
  subatomic.Site.writeAsset(Site.scala:125)
  subatomic.Site.$anonfun$buildAt$1(Site.scala:85)
  subatomic.Site.$anonfun$buildAt$1$adapted(Site.scala:82)
  scala.collection.immutable.Vector.foreach(Vector.scala:1856)
  subatomic.Site.buildAt(Site.scala:82)
  subatomic.builders.blog.Blog$.createSite(Blog.scala:308)
  subatomic.builders.blog.Blog$App.main(Blog.scala:87)
  subatomic.builders.blog.Blog$App.main$(Blog.scala:75)
  ammonite.$file.blog$$anon$1.main(blog.sc:11)
  ammonite.$file.blog$.main(blog.sc:39)
  ammonite.$file.blog$$routes$.$anonfun$apply$1(blog.sc:48)
  ammonite.$file.blog$$routes$.$anonfun$apply$1$adapted(blog.sc:48)
  mainargs.Invoker$.$anonfun$invoke0$3(Invoker.scala:59)

Either shading it (undesirable), or relying on something with stronger bincompat guarantees, or just implementing your own for the small subset of operations that we actually need.

Probably the latter. Or better-files, if it's published for Scala 3.

[keynmol]

Ammonite 2.4.0:

java.lang.NoSuchMethodError: 'boolean os.copy$.apply$default$7()'
  subatomic.Site.writeAsset(Site.scala:125)
  subatomic.Site.$anonfun$buildAt$1(Site.scala:85)
  subatomic.Site.$anonfun$buildAt$1$adapted(Site.scala:82)
  scala.collection.immutable.Vector.foreach(Vector.scala:1856)
  subatomic.Site.buildAt(Site.scala:82)

2.3.8:

Compiling /Users/velvetbaldmime/projects/blog/blog.sc
java.lang.NoSuchMethodError: 'void upickle.core.Types$CaseW.writeSnippet$(upickle.core.Types$CaseW, scala.Function1, upickle.core.ObjVisitor, java.lang.String, upickle.core.Types$Writer, java.lang.Object)'
  subatomic.search.SearchIndex$$anon$12.writeSnippet(SearchIndex.scala:135)
  subatomic.search.SearchIndex$$anon$12.writeToObject(SearchIndex.scala:135)
  subatomic.search.SearchIndex$$anon$12.writeToObject(SearchIndex.scala:135)
  upickle.core.Types$CaseW.write0(Types.scala:208)
  upickle.core.Types$CaseW.write0$(Types.scala:204)
  subatomic.search.SearchIndex$$anon$12.write0(SearchIndex.scala:135)

[keynmol]

For now, I dropped os-lib down to 0.7.2 for anything but Scala 3. Ammonite's latest depends on 0.7.2, so this keeps the binary compatibility issue manageable.

Scala 3 version is still unusable without scalatags, so it doesn't matter much that dependencies diverge.