Remove str from build.CustomTarget sources

[bonzini]

This is inspired by #14885, which reported that a source of type str to a custom target would be exported to the introspection data as os.path.join(meson_root, relative_path).

However, considering that the testcase for it had to use i18n.merge_file, I checked instead whether str sources could be removed altogether. It seems like they can, and the hardest part is actually getting mypy to comply.

I'm still not 100% sure about the handling of the get_outputs() case in the qt module, so I'm marking this as draft until I write a testcase for that.

Fixes: #2748

[bonzini]

#9012 never worked for compile_resources because the call to _parse_qrc_deps fails to find the generated file.

This can be seen with this test:

diff --git a/test cases/frameworks/4 qt/subfolder/copyfile.py b/test cases/frameworks/4 qt/subfolder/copyfile.py
new file mode 100644
index 000000000..ff42ac359
--- /dev/null
+++ b/test cases/frameworks/4 qt/subfolder/copyfile.py	
@@ -0,0 +1,6 @@
+#!/usr/bin/env python3
+
+import sys
+import shutil
+
+shutil.copyfile(sys.argv[1], sys.argv[2])
diff --git a/test cases/frameworks/4 qt/subfolder/meson.build b/test cases/frameworks/4 qt/subfolder/meson.build
index f1b84e6ae..2295b1993 100644
--- a/test cases/frameworks/4 qt/subfolder/meson.build	
+++ b/test cases/frameworks/4 qt/subfolder/meson.build	
@@ -30,3 +30,8 @@ endif
 
 app = executable('subfolder', 'main.cpp', qresources, dependencies : qtdep, cpp_args: extra_cpp_args)
 test(qt + 'subfolder', app)
+
+# Test compile_resources with a custom_target source, to ensure that
+# custom target outputs are correctly converted to File objects.
+ct_qrc = custom_target('stuff3_ct.qrc', output: 'stuff3_ct.qrc', input: 'resources/stuff3.qrc', command: ['python3', files('copyfile.py'), '@INPUT@', '@OUTPUT@'])
+qtmodule.compile_resources(sources : ct_qrc, method : get_option('method'))