bug: allow specifying ref/path as an alias

[jdx]

this also allows aliasing prefix: and system. (no idea why one might want to alias system, but I figure we can allow it)

I noticed this thread and realized people need to alias these as well:

• Expand instructions for using custom-compiled elixir asdf-vm/asdf-elixir#63

[codecov]

Codecov Report

Base: 81.92% // Head: 82.09% // Increases project coverage by +0.17% 🎉

Coverage data is based on head (4f58375) compared to base (ad88f62).
Patch coverage: 95.34% of modified lines in pull request are covered.

❗ Current head 4f58375 differs from pull request most recent head afd88a7. Consider uploading reports for the commit afd88a7 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #166      +/-   ##
==========================================
+ Coverage   81.92%   82.09%   +0.17%     
==========================================
  Files          90       90              
  Lines        2854     2860       +6     
==========================================
+ Hits         2338     2348      +10     
+ Misses        516      512       -4     

Impacted Files	Coverage Δ
src/cli/alias/get.rs	100.00% <ø> (ø)
src/cli/alias/ls.rs	88.88% <ø> (ø)
src/cli/alias/set.rs	100.00% <ø> (ø)
src/cli/alias/unset.rs	100.00% <ø> (ø)
src/cli/env.rs	100.00% <ø> (ø)
src/cli/global.rs	100.00% <ø> (ø)
src/cli/install.rs	81.08% <ø> (ø)
src/cli/local.rs	76.92% <ø> (ø)
src/cli/mod.rs	87.69% <ø> (ø)
src/cli/plugins/ls.rs	100.00% <ø> (ø)
... and 10 more

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