confusing build error output

When I ran dnt build with "test" set to "true", I ended up getting the following error message no matter what I changed about my build configuration:

```
[dnt] Type checking ESM...
error TS2209: The project root is ambiguous, but is required to resolve export map entry '.' in file '/Users/brianleonard/psych-ds/clean/psychds-validator/npm/package.json'. Supply the `rootDir` compiler option to disambiguate.

Build failed with error: Error: Had 1 diagnostics.
at getProgramAndMaybeTypeCheck (https://jsr.io/@deno/dnt/0.41.3/mod.ts:468:17)
at build (https://jsr.io/@deno/dnt/0.41.3/mod.ts:354:17)
at eventLoopTick (ext:core/01_core.js:168:7)
at async file:///Users/brianleonard/psych-ds/clean/psychds-validator/build_npm.ts:36:5
Stack trace: Error: Had 1 diagnostics.
at getProgramAndMaybeTypeCheck (https://jsr.io/@deno/dnt/0.41.3/mod.ts:468:17)
at build (https://jsr.io/@deno/dnt/0.41.3/mod.ts:354:17)
at eventLoopTick (ext:core/01_core.js:168:7)
at async file:///Users/brianleonard/psych-ds/clean/psychds-validator/build_npm.ts:36:5
```

Eventually, I looked into the source code and realized that "rootDir" is not a valid compilerOption for dnt build, and that the actual value I needed to change was "rootTestDir".

This error message strikes me as very misleading, and personally led me to about half a day of hopeless debugging and frustration. I think the error message should do something to indicate that adding "rootTestDir" is the actual solution.