Add BuildTarget.baseDirectory: Uri

Following up from discussions in #30

Currently, there is no way to associate a BuildTarget with a single directory. This information is needed by some IDEs like IntelliJ in order to group together related targets such as main and test.

Based on discussions in #30, we should add a new field to the BuildTarget data structure

trait BuildTarget {
+ /** The directory where this target belongs to. Multiple build targets are allowed to map
+  * to the same base directory. A base directory does not determine the sources 
+  * of a target, see buildTarget/sources.
+ def baseDirectory: Uri
}

At the moment, the IntelliJ Scala plugin infers the base directory from BuildTargetIdentifier.uri path ignoring the URI fragment/query-parameters. Build target URIs are supposed to be opaque IDs, which the build tool is free to encode in any way it likes.

[jvican]

I’m fine with moving forward with the current proposal. Let’s try to get the BSP v2.0 release out soon 😺

[jvican]

Fine, but I’m not comfortable with the current inconsistency in the protocol. If you want to add baseDirectory: Uri to BuildTarget, I propose you change TextDocumentIdentifier to TextDocument and only leave the suffix Identifier for something that is required to be unique, but that requires no assumptions on the client side. Otherwise, we should keep it as is.