Pkg: Check that specfile name matches package when loading spec

We only check the first part of the filename, so filenames
such as 'foo.spec.in', which must be preprocessed before use,
will be accepted.

Signed-off-by: Euan Harris <euan.harris@citrix.com>
Merge pull request #379 from euanh/spec-wrapper

Pkg: Check that specfile name matches package when loading spec