Remove leading slash from paths in Babel options

[klaussner]

Removes the leading slash from the filename and sourceFileName Babel options so that Babel plugins can distinguish relative paths from absolute paths.

I couldn't find out if the filename property is used somewhere in the Meteor build process but the sourceFileName is used in tools/isobuild/bundler.js where the removed slash is handled correctly.

Fixes #8566

[benjamn]

Do these paths get used in the source maps generated by Babel? I wonder if absolute vs. relative paths could be an issue there?

[klaussner]

Yes, the sourceFileName is the path that will end up in the sources array of the source map, so that one must be relative.

[benjamn]

I vaguely remember a bug when loading a source map on a page like https://example.com/nested/query/string. In the Dev Tools, paths without leading /s get interpreted relatively, e.g. packages/whatever.js would be displayed as /nested/query/string/packages/whatever.js, which is a little confusing.

Maybe we can just modify the paths in sources to add back the leading /s?

[klaussner]

I think the source maps generated by BabelCompiler are never served directly to the client (or used for server-side debugging). Instead, meteor://💻app is prepended to the sourceFileName and the result is always meteor://💻app/packages/whatever.js, regardless of whether there is a leading /.