qmake plugin ignores source-subdir parameter?
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Issue body
### Bug Description
Looks like the "source-subdir" is being ignored for the qmake plugin (snapcraft 8.5.1) here.
It looks for the project file in the src dir, not the subdir.
Setting the "qmake-project-file" to the file doesn't work either, only including the subdir path sources/project.pro works.
Is this a bug or am I doing something wrong?
Thanks!
### To Reproduce
Try to build a minimal project.
### part yaml
```shell
parts:
test:
plugin: qmake
source: https://<some git repo>
source-type: git
source-subdir: sources
qmake-major-version: 6
(qmake-project-file: project.pro
```
### Relevant log output
```shell
Without qmake-project-file:
Cannot read /root/parts/test/src: file to open is a directory
With qmake-project-file:
Cannot find file: /root/parts/test/src/project.pro
```
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