Spec: fix handling of multiple .spec files (issue #3)

Instead of assuming any existing .spec file is the One,
we now check any .spec has matches the package name
(depending on hasExec or hasLib).

Previously we had (issue 3):

$ mkdir tmp
$ cd tmp
$ cblrpm spec pkg1
$ cblrpm spec pkg2
[pkg1].spec already exists creating [pkg2].spec.cblrpm

(When two or more spec files existed it was ok.)

Thanks to @yaccz for the report.