manifest: add ports-mgmt/pkg_tree

[grahamperrin]

In pinned #141 and elsewhere across https://github.com/helloSystem/, too many discussions lack an essential understanding of dependencies.

pkg_tree https://www.freshports.org/ports-mgmt/pkg_tree/ gives you an ascii-graphical-tree overview of dependencies of a port. Make this integral to helloSystem, then promote use of things such as this.

[probonopd]

Thanks @grahamperrin. The build failures are my fault. Will have to fix them.

[probonopd]

Please add ports-mgmt/pkg_tree to packages.hello instead.

Reason:

• Thoughts on the FreeBSD ports/package system hello#161 (comment) - adding deps to the MANIFEST broke the ISO build, because apparently pkg add cannot install dependencies whereas pkg install requires the pkg to be installed to be in a repository, which we don't have

[grahamperrin]

Superseded by #406