dap: Centralise debug launch path resolution

[charlieegan3]

Related to #2018 (comment)

When a user starts a debug session, three path values reach OPA's debugger — inputPath,
bundlePaths, dataPaths.

bundlePaths, dataPaths come from the extension:

• https://github.com/open-policy-agent/vscode-opa/blob/247590d/src/da/activate.ts#L105-L108
• https://github.com/open-policy-agent/vscode-opa/blob/247590d/src/opa.ts#L8-L18

See also https://github.com/open-policy-agent/vscode-opa/blob/247590dbdca7e024e0f7a0f8ea7a9a8ca7454f59/src/da/activate.ts#L116-L122

inputPath comes from regal

• regal/internal/lsp/server_command.go

  Lines 143 to 151 in 118f8d3

  	responseParams := map[string]any{
  	"type": "opa-debug",
  	"name": args.Query,
  	"request": "launch",
  	"command": "eval",
  	"query": args.Query,
  	"enablePrint": true,
  	"stopOnEntry": true,
  	"inputPath": inputPath,

Ideally, all would be determined by Regal. This should be doable with the config discovery code we already have.