Support JEP-261 Module System

[retronym]

(Part of the epic, #139)

• Expose new command line options to scalac (and later to the scala script runner) (--module-path, etc)
• Build atop StandardJavaFileManager and java.lang.module._, to process these options and create a ClassPath implementation and resolved module graph
• Update Context.isAccessible to check for reads/exports edges between the call site and referenced symbol.
• To support mixed compilation, add a parser for module-info.java, and feed a descriptor of the module descriptor to the module graph configuration.

WIP: https://github.com/retronym/scala/tree/topic/jpms

% qscalac  -nobootcp -modulepath ""  $(f "class C { def foo = javax.xml.bind.DatatypeConverter.printBase64Binary _ }")
/tmp/a.scala:1: error: object DatatypeConverter is not a member of package javax.xml.bind
class C { def foo = javax.xml.bind.DatatypeConverter.printBase64Binary _ }
                                   ^
one error found

% qscalac  -nobootcp -modulepath "" -addmodules:java.xml.bind  $(f "class C { def foo = javax.xml.bind.DatatypeConverter.printBase64Binary _ }")

% qscalac  -nobootcp -modulepath "" $(f "class C { sun.reflect.misc.ConstructorUtil.getConstructors(null) }")
/tmp/a.scala:1: error: class ConstructorUtil in package misc cannot be accessed in package sun.reflect.misc
class C { sun.reflect.misc.ConstructorUtil.getConstructors(null) }
                           ^
one error found

% qscalac  -nobootcp -modulepath "" -addexports:java.base/sun.reflect.misc=ALL-UNNAMED  $(f "class C { sun.reflect.misc.ConstructorUtil.getConstructors(null) }")

Non Goals

Support for --module-source-path. A batch of source files will be associated with at most one module.

Open Issues

How should we structure our build to allow us to call Java 9 APIs from this code path? I'm thinking we do this from a small pure-Java project and incorporate the results into scala-reflect.jar, similar to how we've dealt with scala-asm.

[Knut-Fjellheim]

Is there a target date, or target version, for this feature?

[retronym]

I hope its ready for Scala 2.13.0 later this year. If not, then we'll make sure we at least have the plumbing installed to add it for 2.13.1.

[retronym]

This test in my WIP branch shows what I've got working so far.

[xuwei-k]

duplicate? #482

[adriaanm]

Thanks, closed the other one :-)