Implement `pip` and `junction` origin loading for SourceMirror plugins

[gtristan]

This adds support for loading SourceMirror plugins via both pip and junction plugin origins.

This tests essential mirror behavior when loading the plugins with pip or junction plugin origins, and also tests the case where the mirror itself is used to define the mirror required to obtain itself across a junction boundary, succeeding when the mirror is not used and asserting the failure mode when mirroring is forced.

[gtristan]

Note about remaining imperfections.

We do not lazy load the SourceMirror plugins, which means that we load them in a loop, while loading the mirrors configuration in the project configuration, rather than delaying the load until the SourceMirror is actually used.

This can still result in some situations where:

• You have two SourceMirror plugins defined through separate junctions
• You need one of these SourceMirrors in order to access the other SourceMirror through its junction
• The project will not load properly depending on the order in which the SourceMirrors are declared in project.conf

This is a bit tricky to resolve, and I suspect the current functionality to satisfy the majority of use cases, but we will most likely run into this issue.

[juergbi]

The project will not load properly depending on the order in which the SourceMirrors are declared in project.conf

As long as the behavior is deterministic and this results in a clear error message, I don't think we need to let that issue block this PR. It may be worth a note in the documentation. However, I agree that we likely want to eliminate this limitation at some point.