Packaging: Add directories and symlinks to deb/rpm packages

[cognifloyd]

This PR is working towards doing packaging via pantsbuild. Eventually, I hope to archive and stop using st2-packages.git.

Previous PRs in this series:

• The pants nfpm backend was activated in Packaging: Inject st2 version into rpm/deb packages (pants-plugins/release) #6321.
• Basic BUILD metadata for deb/rpm packages (nfpm_deb_package and nfpm_rpm_package) was added in Packaging: Add basic BUILD metadata for rpm/deb packages #6325 along with the deb maintainer scripts and rpm scriptlets.
• Packaging: Add systemd files to rpm/deb packages #6326 added the systemd files (units and generators) to the deb/rpm packages

This PR defines the metadata for the directories and symlinks that the deb/rpm packages should install. To do this, we need to add nfpm_content_directories and nfpm_content_symlinks to BUILD metadata and then make the nfpm_*_package targets depend on them.

Directories are defined st2-packages.git here:

• https://github.com/StackStorm/st2-packages/blob/master/packages/st2/debian/st2.dirs
• https://github.com/StackStorm/st2-packages/blob/master/packages/st2/rpm/st2.spec#L124-L132

Symlinks are defined in st2-packages.git here:

• https://github.com/StackStorm/st2-packages/blob/master/packages/st2/debian/st2.links

[cognifloyd]

This is defined in https://github.com/StackStorm/st2-packages/blob/master/packages/st2/debian/st2.dirs but I don't think they're necessary. When we add the files that go in these directories, the directories will be implicitly added, and we don't have specific permissions to set on these directories as we assume they will be created by the relevant package that owns the directory.

So, I left it commented out. I can either remove them now, or we can remove them once the files that go in these directories get registered (in a follow-up PR).

[cognifloyd]

https://github.com/StackStorm/st2-packages/blob/d4d2d8dfdf1c88412e5d58635adb87da9c671952/packages/st2/debian/preinst#L24-L26

https://github.com/StackStorm/st2-packages/blob/d4d2d8dfdf1c88412e5d58635adb87da9c671952/packages/st2/rpm/preinst_script.spec#L9-L11

[cognifloyd]

https://github.com/StackStorm/st2-packages/blob/d4d2d8dfdf1c88412e5d58635adb87da9c671952/packages/st2/debian/preinst#L27-L29

https://github.com/StackStorm/st2-packages/blob/d4d2d8dfdf1c88412e5d58635adb87da9c671952/packages/st2/rpm/preinst_script.spec#L12-L14

[cognifloyd]

Each set of directories above (:root_dirs, :packs_group_dirs, :st2_user_dirs) defines the relevant permissions. They are separate targets because those permissions differ. Then, this target makes it easy to depend on all of the directories in the nfpm_*_packages targets.