inferExports = (descriptor, conditions, types) =>
  fromEntries(inferExportsEntries(descriptor, conditions, types))
inferExports reads a package.json (package descriptor) and constructs a map
of all the modules that package exports.
The keys are the module specifiers for the module map of any package that
depends upon this one, like `semver` for the main module of the `semver`
package.
The values are the corresponding module specifiers in the dependency
package's module map, like `./index.js`.

@param {PackageDescriptor} descriptor - the parsed body of a package.json file.
@param {Set<string>} conditions - build conditions about the target environment
for selecting relevant exports, e.g., "browser" or "node".
@param {LanguageForExtension} types - an object to populate
with any recognized module's type, if implied by a tag.
@returns {Record<string, string>}