1. Disallow the addition of multiple "provides" dependencies to a package (enforce the
"one package/one service" paradigm).
2. Only allow "requires" and "extends" of installed and enabled packages - when you build a
package interactively in the APM, it installs it, and would allow you to require packages
that aren't installed, which of course would make your package fail heavily if you
mounted or otherwise tried to use it.
3. Make adding "extends" properly copy parameters of its parent package.
4. When adding a package, rebuild the cached package dependency information (added by
me in 5.5.0, I'd missed the need to rebuild the cache in the APM interactive package
builder).
