Allow QML plugin to specify import name explicitly

For creating the qmlplugindump rule we need the name of the import. So
far we're trying to derive this from the directory name, but that
becomes complicated if versions are included, like

  QtQuick/Controls.2/Fusion

Instead of trying to tweak the regexp even more to capture this, we
now allow a plugin to set it's name explicitly via IMPORT_NAME.

Change-Id: Ie75dae7d41398b3ac19ccb6910002b6fad009891
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>

mkspecs/features/qml_plugin.prf

@@ -94,7 +94,7 @@ load(qt_common)
         }
 
         load(resolve_target)
-        TARGETPATHBASE = $$replace(TARGETPATH, \\.\\d+\$, )
+        isEmpty(IMPORT_NAME): IMPORT_NAME = $$replace(TARGETPATH, \\.\\d+\$, )
 
         !qml1_target {
             isEmpty(QMAKE_PLUGINDUMP_DEPENDENCIES_FILE):exists($$_PRO_FILE_PWD_/dependencies.json): \
@@ -104,7 +104,7 @@ load(qt_common)
         }
 
         qmltypes.target = qmltypes
-        qmltypes.commands = $$QMLPLUGINDUMP -nonrelocatable $$QMAKE_QMLPLUGINDUMP_FLAGS $$replace(TARGETPATHBASE, /, .) $$IMPORT_VERSION > $$QMLTYPEFILE
+        qmltypes.commands = $$QMLPLUGINDUMP -nonrelocatable $$QMAKE_QMLPLUGINDUMP_FLAGS $$replace(IMPORT_NAME, /, .) $$IMPORT_VERSION > $$QMLTYPEFILE
         qmltypes.depends = $$QMAKE_RESOLVED_TARGET
     } else {
         qmltypes.CONFIG += recursive