Resource leak: Sources not closed

[rethab]

There are several places in this repo where Source.fromFile is used without closing the source thereafter. While this is likely not a problem in the productive sources (see output below), this has in the past led to issues with running tests on windows.

Note that this issue was originally reported by @michelou in the dotty repo scala/scala3#8358 and @smarter found that this repo has the same issues.

Occurences

note that I excluded Using here because it is actually used in one place

[rethab:~/dev/scala] 2.13.x ± λ git grep "Source.fromFile" *.scala | grep -v Using
project/GenerateFunctionConverters.scala:    val x = scala.io.Source.fromFile(f)
src/library/scala/concurrent/package.scala: *   val source = scala.io.Source.fromFile("/etc/dictionaries-common/words")
src/partest/scala/tools/partest/nest/FileManager.scala:    compareContents(io.Source.fromFile(original).getLines.toSeq, io.Source.fromFile(revised).getLines.toSeq, original.getName, revised.getName)
test/benchmarks/src/main/scala/scala/collection/immutable/GenerateVectorBenchmark2Charts.scala:    Source.fromFile(file, "UTF8").getLines().drop(1).map { s =>
test/files/jvm/t2104.scala:    g(Source.fromFile(f))
test/files/run/t4671.scala:val s = scala.io.Source.fromFile(o.file)
test/files/run/t4671.scala:val s = scala.io.Source.fromFile(o.file)
test/files/run/t6240-universe-code-gen.scala:  val actual = scala.io.Source.fromFile(actualFile)
test/files/specialized/fft.scala:    val f = io.Source.fromFile(inputFileName)

[som-snytt]

#7244 and relatedly scala/scala#8887

[som-snytt]

The PR reduces some usages. There are old tickets along similar lines, including the API is unexpected or hard to use, please at least update the docs. Now just Files.readString or reflect.io.Path("f").toFile.slurp().