Handle modulepath: file imports

[thomasquinn]

This PR adds support for resolving modulepath: URIs using the module paths declared in a PklProject's evaluatorSettings, as the CLI does. This is a different approach than #93, which instead adds an LSP setting for these paths. Let me know if you'd prefer that approach over using evaluatorSettings.modulePath. I thought that using the standard PklProject setting may avoid a bit of editor setup for users and allow the LSP to match the CLI's behavior more closely.

This is my first time writing Kotlin, so please tell me if there's a better way of expressing this code. In particular, I can't tell if a more functional or imperative style is typical here.

My use case for this is to have workspace root-relative imports for generated Pkl files in Bazel. As a demonstration, here is the behavior of pkl-vscode (v0.21.0, unchanged) before and after this patch to pkl-lsp, with the following PklProject:

amends "pkl:Project"

evaluatorSettings {
  modulePath {
    "."
    "./bazel-bin"
  }
}

Before ("Cannot resolve import" error on amends):

After ("Go to definition" jumpts to Example.pkl within bazel-bin):

This does not completely implement modulepath:, as it makes no attempt to read files within JARs or Zip files, which the spec (and CLI) supports.

Closes #91.