Eclipse plugin: implementing package spec proposal

[sethladd]

Tracking issue for implementing the package spec proposal.

Main issue: dart-archive/dart_enhancement_proposals#5

Proposal: https://github.com/lrhn/dep-pkgspec/blob/master/DEP-pkgspec.md

[sethladd]

Marked this as blocking #23372.

[sethladd]

Not sure what changes are required to Eclipse plugin, if any, to compensate for the package spec functionality.

tldr: symlinks going away.

[clayberg]

cc @danrubel.
cc @scheglov.
cc @devoncarew.
Set owner to @keertip.
Added this to the 1.11 milestone.
Removed Priority-Unassigned label.
Added Priority-High label.

[lrhn]

Added Area-Editor label.

[sethladd]

Bumped to 1.12, but we need to complete all work on .packages for 1.12, including Eclipse plugin support. Thank you for the help!