Improper handling of source maps with relative source roots

Source maps can have sourceRoot property; it's defined as a string that's prepended to every entry in the sources list. The specification doesn't put any constraints on this field; consequently, this can be a relative directory (eg. ..).

In my understanding, the intended semantics is that it's to be resolved relative to the URL of the source map itself, eg. given { sourceRoot: '..', sources: 'a.js' } and source map at file:///example/source.js.map the resolved source URL should be ../a.js => file:///a.js.

appmap-node does not handle this case correctly. Consider for example typeorm-0.3.20; metadata-args/MetadataArgsStorage.js.map contains:

$ jq '{sources, sourceRoot}' node_modules/typeorm/metadata-args/MetadataArgsStorage.js.map
{
  "sources": [
    "../../src/metadata-args/MetadataArgsStorage.ts"
  ],
  "sourceRoot": ".."
}

However, when instrumenting the file, relevant location.path in transform() (as determined by the debugger) reads nonsensical ../home/divide/projects/tmp/NestJS-TypeORM-Example/node_modules/typeorm/metadata-args/src/metadata-args/MetadataArgsStorage.ts (note the working directory in this run is /home/divide/projects/tmp/NestJS-TypeORM-Example/).

(The path as indicated by the source map would be incorrect in any case (file:////home/divide/projects/tmp/NestJS-TypeORM-Example/src/metadata-args/MetadataArgsStorage.ts by my reading) which seems to be a bug in typeorm build, but that's beside the point.)

Note: I started working on this issue, here's a WIP branch: https://github.com/getappmap/appmap-node/tree/wip/source-map-relative-root

Start in src/hooks/instrument.ts at the transform() location referenced in the issue, then inspect the WIP source-map-relative-root branch and reproduce the TypeORM example. Verify that a relative sourceRoot is resolved against the source map URL rather than the working directory, and confirm the resulting location.path is sensible.