Assume `rootDir` is the current configuration directory

[DanielRosenwasser]

Discussed at #54500:

Currently rootDir, if not specified, is inferred based on the set of root files found (unless composite is set).
In 6.0, the default rootDir will always be the directory containing the tsconfig.json file.
If a non-config-based build occurs, rootDir will be inferred as before.

This lets us go faster because we don't need to compute the set of input files in order to determine the structure of the output directory.

This also allows our language service to trivially determine whether a file could belong to another tsconfig.json with no differences in command-line behavior.

When unspecified, rootDir will be the same as "${configDir}".

Note

This is not the same as the similarly-named option rootDirs, which will continue to work the same in both 6.0 and 7.0.

[DanielRosenwasser]

One thing to note is that for a project setup like this:

project
├── src
│   └── file.ts
└── tsconfig.json

With this change, if rootDir isn't specified, 6.0 and 7.0 will now include a src directory in the output directory (outDir), leaving you with something:

project
├── dist
│   └── src
│       └── file.js
├── src
│   └── file.ts
└── tsconfig.json

So in projects where tsconfig.json lives outside of the main source directory, you will have to specify rootDir as the source directory.