[GR-37095] Adds ability to create jar entries for directories

[zakkak]

Just create an .mxkeep file in a directory and any jar distribution
containing this directory will include a jar entry for it. The .mxkeep
file will not be included.

[zakkak]

@gilles-duboscq @olpaw please review.
I followed the .gitkeep convention and used .mxkeep instead of .empty_dir.

[olpaw]

LGTM

Please also update

version = VersionSpec(...

accordingly in an additional commit.
(Search for # The version must be updated for every PR (checked in CI) in mx.py)

[zakkak]

Please also update

version = VersionSpec(...

accordingly in an additional commit. (Search for # The version must be updated for every PR (checked in CI) in mx.py)

What should I add as the PR reference next to the version? GH-256?

[fniephaus]

What should I add as the PR reference next to the version? GH-256?

Yes, that should work. Thanks!

[olpaw]

What should I add as the PR reference next to the version? GH-256?

Yes, that should work. Thanks!

Even better, use GR-37095 (see #256 (comment))

[zakkak]

Done :)

[dougxc]

@olpaw @fniephaus it's usually better to update version with a separate commit in the internal PR used to integrate a GitHub PR. That way, an external committer does not have to try "win" the version bump race.

[olpaw]

@olpaw @fniephaus it's usually better to update version with a separate commit in the internal PR used to integrate a GitHub PR. That way, an external committer does not have to try "win" the version bump race.

I see. If there is a conflict I will replace @zakkak version update commit with my own.

[dougxc]

For reference, the motivation for this change is here: oracle/graal#4327 (comment)

[gilles-duboscq]

LGTM. Thanks.

[olpaw]

Merged as 39ac3b7