Allow staging part of a source

Background

Sometimes projects are structured in a way that necessitates singling out a subdirectory before building the project. This happens quite frequently in projects that are an amalgamate of various plugins or themes.

Currently, to build one of these from any source besides tar, local or zip, we need to prefix configure-commands with the relevant commands to move this around - be that mv-ing, cp-ing, cd-ing or suchlike.

This can be annoying for elements that would otherwise not have to depend on an element that contains these binaries (this is fairly common for themes).

Task description

To mitigate this, we could make the base-dir API from our zip/git global (and perhaps rename it).

Acceptance Criteria

We should have a way to stage only part of a source in all source plugins.

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