[heft] Fix an issue where heft would crash when copying static assets in --watch mode.

[iclanton]

Summary

There is an issue where heft build --watch will crash when a file listed as a static asset to copy to the "lib" folder(s) is edited. This issue was introduced when support for automatically detecting the "lib" folder specified as the outDir in tsconfig.json was added. That path has to be resolved relative to the tsconfig.json file that specifies it, which may be a file specified in an "extends" property.

Details

This PR adds a resolvedDestinationFolderPaths property to the CopyFilesPlugin copy operation descriptor which is preemptively resolved to an absolute path.

How it was tested

Tested by building a large project containing .scss files both in --watch and not in --watch mode.