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A normal dependency of X on Y consists of a "must-use" constraint ("if X is
loaded, Y must be loaded too") and an "ordering" constraint ("if X and Y are
both loaded, Y must be loaded before X").
The previously-existing support for "unordered" dependencies allows you to
create a dependency with "must-use" but not "ordering". This commit adds "weak"
dependencies, which have "ordering" but not "must-use".
As an example, the accounts-base package wants to define some Handlebars helpers
like {{currentUser}} if the handlebars package is being used, but it's fine to
use accounts-base without handlebars. So it should declare a weak dependency on
handlebars.
A package can tell if another package has already been loaded by checking to see
if `Packages.foo` exists. (So as a result, even slices which export no variables
get a `Packages.foo = {}` line.) Weak dependencies do not import symbols into
your namespace; you must access their symbols through
`Packages.foo.someExportedSymbol`. You don't get to use plugins from your weak
dependency.