Support nested file imports in LSP

[psiemens]

Description

I noticed a small bug that occurs when using nested imports in the VS Code extension (e.g. import a file that imports another file).

Here's an example:

contracts/
  MyContractA.cdc
  MyContractB.cdc
transactions/
  myTransaction.cdc

// transactions/myTransaction.cdc

import MyContractA from "../contracts/MyContractA.cdc"

// contracts/MyContractA.cdc

import MyContractB from "./MyContractB.cdc"

In this case, when resolving the import of MyContractB, the mainPath value is still transactions/myTransaction.cdc, so the computed absolute file path (I call this path normalization) is transactions/MyContractB.cdc, which fails.

This PR updates the import logic to normalize paths before setting the location on the sub-checker for the imported program. This way, each value of checker.Location points to the absolute path for that file. Then, any further nested imports are normalized against checker.Location, allowing for arbitrary depth of imports.

Disclaimer: I have only tested this code with file imports and have not yet tested extensively enough to ensure I didn't introduce a regression with address-based imports. I'll try to find time for that today.

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For contributor use:

• Targeted PR against master branch
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work
• Code follows the standards mentioned here
• Updated relevant documentation
• Re-reviewed Files changed in the Github PR explorer
• Added appropriate labels

[turbolent]

Looks good, thank you for fixing the local file resolution, I hadn't realized it wasn't handling relative paths properly 👍

Just one concern, and there seem to be build errors

[turbolent]

Very nice! Thank you for fixing this 🙏