Support wrap files with meson.build in subdirectory

The Apache Arrow project has recently been adding limited support for Meson. I would love to add this to the wrapdb, but being a multi-project language, the Arrow project does not include meson.build at the root of the source distribution. Instead, the relevant file for the wrapdb is located at cpp/meson.build

Is there a way to support this now without having to generate a new source distribution for the Apache Arrow project? If not, perhaps a wrap file setting like `source_directory` could be supported?

Start with the wrapdb handling for Meson projects and the existing wrap-file behavior when meson.build is not at the source root. Reproduce the Apache Arrow layout, where the relevant file is cpp/meson.build, and check whether a source_directory setting can be supported without changing the source distribution. Done means a supported configuration and coverage for this subdirectory layout.

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