[Development] Qt 5 file hierarchy

[Thiago Macieira]

The Qt 5 file hierarchy upon installation should be:

bin/		- executables run by the user
		- unversioned applications, like assistant, linguist, qdbus,
		  qdbusviewer
		- versioned applications, like qmake[35], qdoc5, qmlviewer5, 
		 maybe moc5, uic5, rcc5
doc/		- gone to share/qt5/doc
examples/ - gone to share/qt5/examples
include/	- versioned include dirs:
	QtCore5 or QtCore-5 or QtCore.5 or Qt5Core/
	[...]
	Qt/ 	- gone, forever, no replacement
imports/	- horribly flawed design, see below
lib/		- arch-specific files (also lib<qual> or lib/<arch>/)
	./		 - versioned libraries (.a, .so, .la, .prl)
	pkgconfig/ - versioned .pc files
	qt5/ 		 - arch-specific support files:
		bin/ or libexec/
		 - executables not run by the user, like syncqt, lrelease, lupdate
		imports/
		 - QtDeclarative imports
		qml2/
		 - QML2 (including QtQuick2) arch-specific imports
		mkspecs/ ?
share/	- arch-independent files
	qml2/	 - QML2 (including QtQuick2) arch-independent imports
	qt5/
		doc/
		examples/
		mkspecs/	?

Note on imports: the design is flawed. It was flawed in Qt 4 and it's worse now 
on Qt 5. For Qt 4, the flaw was that it did not differentiate QML imports that 
were arch-independent from the ones that required plugins. With Qt 5, it 
becomes worse because Qt Quick 1 and 2 share the same directory.

Instead, I recommend we immediately change QML2 to the system that Perl uses: 
put the arch-specific files inside the lib hierarchy, for which distributions 
have already solved the multiarch problem, but put arch-independent files in 
the share hierarchy.

In addition, the loader should be clever to merge similar names from the two 
different paths. That is, imagine a .qml file in share/qml2 that requires a 
plugin: if the loader is a 32-bit application, it would search for the plugin 
in lib/qt5/qml2, but if it's a 64-bit application it would search in 
lib64/qt5/qml2.

Additionally, if we're still using QML2 by the Qt 6 release, the plugins could 
be made available in lib/qt6/qml2.

As for mkspecs, I believe they should be in share, since they are technically-
speaking arch-independent.

-- 
Thiago Macieira - thiago.macieira (AT) intel.com
  Software Architect - Intel Open Source Technology Center
     Intel Sweden AB - Registration Number: 556189-6027
     Knarrarnäsgatan 15, 164 40 Kista, Stockholm, Sweden
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