Allow multiple locations for modules in tests

[GuntherRademacher]

The module location logic used by the QT4 test runner currently supports only a single location for a module. But some tests use multiple locations. The changes in this PR add support for this.

This fixes QT4 test cases modules-collide-var-001 and modules-collide-fn-001.

Per 5.12 Module Import, a module import may use the same prefix and URI as the module declaration of the module that it occurs in:

The namespace prefix specified in a module import must not be xml or xmlns [err:XQST0070], and must not be the same as any namespace prefix bound in the same module by another module import, a schema import, a namespace declaration, or a module declaration with a different target namespace [err:XQST0033].

The check for duplicate namespace prefixes has been refined accordingly.

This fixes QT4 test case XQST0093a.