Allow globs in workspace definitions

[sigurdm]

Part of #4127

Initially we thought we would not allow globs, because resolving them would be on the hot path of ensureResolutionUpToDate and thus dart run.

But I think we are getting close to the conclusion that the fast path of ensureResolutionUpToDate only should depend on .dart_tool/package_config.json, not parsing and resolving pubspec.yamls - thus we can allow globs.

There is still the issue that executableForCommand needs to know if a package is a direct dependency or not. We might want to solve that issue before landing this. Discussing in #4222

[sigurdm]

We've decided not to do this in the first incantation of workspaces.