Include the library path on missing imports

[SquidDev]

• Importers now return a "SearchedIn" result, which details where we attempted to load a file from. This is then printed in error messages.

• Refactor some common library loading code into Frontend.Driver from the LSP server and main compiler driver.

Example output:

tests/amc/files/import.ml[1:6 ..1:23]: error (E1010)
  Cannot resolve "my_lib.ml"
  │ 
1 │ open import "my_lib.ml"
  │      ^^^^^^^^^^^^^^^^^^

  Searched in:
    • /home/squiddev/programming/amulet/lib/my_lib.ml
    • /home/squiddev/programming/amulet/.stack-work/blah/blugh/lib/my_lib.ml
    • /home/squiddev/programming/amulet/.stack-work/blah/lib/my_lib.ml

[SquidDev]

Mostly done so I could actually understand why my lsp configuration was broken. But I still think it's useful.

[SquidDev]

Tests should be working as of 26ec59db6b6c9e45974ce2e779f572bf4b282e59 - it's not a problem with this PR.