Fix file globs for relative paths

[dsherret]

Instead of doing:

project.getSourceFiles("**/src/**/*.ts")

Should be able to do:

project.getSourceFiles("src/**/*.ts")
// or
project.getSourceFiles("./src/**/*.ts")
// or
project.getSourceFiles("../otherFolder/**/*.ts") // didn't test, but I think it doesn't work

[dsherret]

addExistingSourceFiles works (for non-virtual file systems). It's actually getSourceFiles with a glob that doesn't. I've updated above.