Can we eliminate perennial-alias and allow perennial to depend on phet-core via a parallel checkout?

[samreid]

Discovered in phetsims/chipper#1464

One idea @jonathanolson and @zepumph and I discussed today is:

• Get rid of perennial-alias
• Change the perennial tooling so that it operates on a separate parallel working copy rather than in-place

perennial-alias has been a point of complexity and confusion, and would be good to eliminate. We discussed that developers could have one working copy which is always on main, and another which is allowed to operate on shas, branches, etc.

However, this would:
a) be a huge development effort since perennial has many complex tasks that currently assume they can operate locally
b) would introduce complexity for sim developers who would need to check out 2x trees of everything and know when to run tasks from phet/checkouts/main vs phet/checkouts/working

I just wanted to write the idea down, knowing that it is likely too expensive to implement. But it is the first viable alternative to perennial-alias that we have considered, which would also allow perennial to use code from phet-core or axon, etc.