Allow updating of migrated outpack archives

[richfitz]

This makes two changes to allow updating of a migrated orderly archive:

• use a link-based file store; this means that we don't require two full copies of the data always (which would be problematic on montagu)
• allow the destination to exist, so long as it looks like something we could migrate into

[codecov]

Codecov Report

Base: 100.00% // Head: 100.00% // No change to project coverage 👍

Coverage data is based on head (1e0a2b2) compared to base (79a012f).
Patch coverage: 100.00% of modified lines in pull request are covered.

❗ Current head 1e0a2b2 differs from pull request most recent head ea62184. Consider uploading reports for the commit ea62184 to get more accurate results

Additional details and impacted files

@@            Coverage Diff            @@
##              main        #2   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         3    +1     
  Lines          113       129   +16     
=========================================
+ Hits           113       129   +16     

Impacted Files	Coverage Δ
R/migrate.R	100.00% <100.00%> (ø)
R/store.R	100.00% <100.00%> (ø)

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