Improve IO.toFile for Windows (URI has an authority component)

[eed3si9n]

The URL encoding of Windows path that works out of the box is:

• file:///C:/Users/foo for path on the local machine.
• file:////unc/Users/foo for UNC path.

Even thought this is not the most modern/canonical encoding, it allows file:/// to be used as a cross-platform prefix file:///${user.home} since on Mac and Linux user.home would look like /Users/foo, it would come out as file:////Users/foo.

original PR

This adds support to convert from an URL to a File on Windows in several ways.

scala> val u0 = new URL("file:C:\\Users\\foo/.sbt/preloaded")
u0: java.net.URL = file:C:/Users/foo/.sbt/preloaded

scala> toFile(u0)
res0: java.io.File = C:\Users\foo\.sbt\preloaded

scala> val u1 = new URL("file:/C:\\Users\\foo/.sbt/preloaded")
u1: java.net.URL = file:/C:/Users/foo/.sbt/preloaded

scala> toFile(u1)
res2: java.io.File = C:\Users\foo\.sbt\preloaded

scala> val u2 = new URL("file://unc/Users/foo/.sbt/preloaded")
u2: java.net.URL = file://unc/Users/foo/.sbt/preloaded

scala> toFile(u2)
res4: java.io.File = \\unc\Users\foo\.sbt\preloaded

scala> val u3 = new URL("file:///C:\\Users\\foo/.sbt/preloaded")
u3: java.net.URL = file:/C:/Users/foo/.sbt/preloaded

scala> toFile(u3)
res5: java.io.File = C:\Users\foo\.sbt\preloaded

scala> val u4 = new URL("file:////unc/Users/foo/.sbt/preloaded")
u4: java.net.URL = file:////unc/Users/foo/.sbt/preloaded

scala> toFile(u4)
res8: java.io.File = \\unc\Users\foo\.sbt\preloaded

This is an improvement over the constructor of File, which is unable to handle u0 and u2 cases:

scala> val u0 = new URL("file:C:\\Users\\foo/.sbt/preloaded")
u0: java.net.URL = file:C:/Users/foo/.sbt/preloaded

scala> new File(u0.toURI)
java.lang.IllegalArgumentException: URI is not hierarchical
        at java.io.File.<init>(File.java:418)

scala> val u2 = new URL("file://unc/Users/foo/.sbt/preloaded")
u2: java.net.URL = file://unc/Users/foo/.sbt/preloaded

scala> new File(u2.toURI)
java.lang.IllegalArgumentException: URI has an authority component
        at java.io.File.<init>(File.java:423)

The error case for two slashes "URI has an authority component" shows up in many cases, most recently borking 0.13.14 in #3086. Note that it might be intuitive to encode file path by starting with file:// similar to http://, however this is specified by Microsoft to be an UNC path, so the URI file://C:/Users/foo/.sbt/preloaded is illegal. Since the authority component after //, in this case C: should denote the remote host name.

This complicates trying to specify sbt global base (default ~/.sbt/) path in a cross platform way.

  local-preloaded-ivy: file://${sbt.preloaded-${sbt.global.base-${user.home}/.sbt}/preloaded/}, [organization]/[module]/[revision]/[type]s/[artifact](-[classifier]).[ext]
  local-preloaded: file://${sbt.preloaded-${sbt.global.base-${user.home}/.sbt}/preloaded/}

The above original is incorrect because on Windows, ${user.home} would return C:\Users\foo\. The solution is to encode without the prefix slashes like u0.

  local-preloaded-ivy: file:${sbt.preloaded-${sbt.global.base-${user.home}/.sbt}/preloaded/}, [organization]/[module]/[revision]/[type]s/[artifact](-[classifier]).[ext]
  local-preloaded: file:${sbt.preloaded-${sbt.global.base-${user.home}/.sbt}/preloaded/}

Fixes #3086
Fixes #2150

[fommil]

I hit this so much, I solved it in class-monkey, you should have asked :-P

[fommil]

FYI https://github.com/fommil/class-monkey/blob/master/src/main/java/fommil/ClassMonkeyUtils.java#L99 and https://github.com/ensime/ensime-server/blob/2.0/util/src/main/scala/org/ensime/util/ensimefile.scala#L51