The `--default-mirror` comand line option is not very useful

This option only controls the default mirror of the toplevel project.

This is increasingly meaningless as projects become modular, or thin application projects are based on larger runtime projects.

We should either remove this command line option altogether, along with an implementation of [this proposal](https://lists.apache.org/api/source.lua?id=1w3s72q4yf23rx0yc57tw0lf94q3g5bj), or we should elaborate on the option such that it can declare a default mirror for all projects loaded in the build pipeline, e.g.:

```
bst --default-mirror gnome korea --default-mirror freedesktop-sdk korea fetch target.bst
```

I think it would be acceptable to just remove the option as a first step and implement it more flexibly on demand in a later stable release of bst 2

Start by locating the existing `--default-mirror` command-line handling and review the linked proposal. Determine whether the first step is to remove the option or support defaults for every project loaded in the build pipeline; done means the chosen behavior is implemented consistently and the obsolete alternative is addressed.

Written by the indexing model from the issue text.