Rename Divisor package to WeilDivisors

[mahrud]

With permission from the authors, I renamed the Divisor package to WeilDivisors to prevent the conflict caused by #3543. I also added a warning so that anyone using the old name sees this warning:

i1 : needsPackage "Divisor"
 -- warning: the 'Divisor' package has been renamed as 'WeilDivisors'.

o1 = WeilDivisors

o1 : Package

Later, we can change this to be an error.

• renamed 'Divisor' package as 'WeilDivisors'
• added deprecation warning for renamed packages
• fixed a dead link in ExteriorIdeals

(Karl: feel free to review only the changes in your packages -- I've asked Doug to review the changes in Core)

[d-torrance]

It would be good if this worked with needsPackage too. Right now, we get the following, which I didn't expect:

i1 : needsPackage "Divisor"
 -- warning: the 'Divisor' package has been renamed as 'WeilDivisors'.

o1 = WeilDivisors

o1 : Package

i2 : needsPackage "Divisor"
 -- warning: the 'Divisor' package has been renamed as 'WeilDivisors'.
../../packages/WeilDivisors.m2:3:10:(3):[11]: error: package WeilDivisors not reloaded; try Reload => true

[mahrud]

The fix is very minimal, so I pushed a commit with [skip ci].

[d-torrance]

The Core changes look good to me!