Paths of released source maps refer to relative paths that don't exist

[bryanforbes]

Bug

After building the source, the sources entries in the generated source maps point to source files. For example, after building @dojo/core/lang, the sources entry will be "sources":["../../../src/lang.ts"]. This is fine for development (since the files exist), but once the project is released, the files no longer exist and the dev tools locate the virtual files in strange places. For the release source maps, the sources entry should be corrected to simply be the original filename so that dev tools think the source files are siblings to the JS files.