mx: "path" and "sourcePath" attributes

[Doug Simon]

The "path" and "sourcePath" attributes for libraries[1] and distributions[2] are now optional. For libraries, the downloaded artifacts are now referenced directly at their location in the mx cache (i.e., ~/.mx/cache). For distributions, the generated jars are now placed in mxbuild/dists/ in the top level directory of their suite. I’ve converted the graal-jvmci-8, graal-compiler and truffle repos to remove the explicit specification of the "path" and "sourcePath” attributes. If you were using the explicitly specified paths for distributions outside of mx, you need to adapt for the extra mxbuild/ in the relative paths.

For libraries, this change avoids the need for creating symlinks/copies in a lib/ directory for suite to artifacts that exist in the mx download cache. For distributions, it allows mx to better control where the generated jars are placed. In particular, it’s easier to avoid putting them into the build/ directory which in the case of graal-jvmci-8 can cause conflicts with HotSpot’s make system.

-Doug

[1] https://bitbucket.org/allr/mx/commits/62d56a8cd479fc0bc8d0fcc0f88cddb862248385
[2] https://bitbucket.org/allr/mx/commits/78ca40794884c0a2c882667a61d8fbd53b7b156f