Expose the location of the modules directory

Add a modulesdir variable to shtk.pc so that third-party packages can
install shtk-compliant modules into the shared directory.  This variable
is also exposed by the SHTK_CHECK macro of shtk.m4 as SHTK_MODULESDIR.
Add the SHTK_MODULESPATH variable

SHTK_MODULESPATH is a user-defined variable that lists the location
of the directories that contain modules.  Useful for third-party
packages that provide their own modules and need to access them
for testing before installing them.
Make import of modules really idempotent

Fix a bug introduced in the previous commit that caused multiple
inclusions of the same module to not be idempotent under some
circumstances.  In particular, multiple inclusion was not detected
when the same module was imported more than once and, in between
imports, another different module was imported.