`src.rpm`-related info support

Although TAG_SOURCEPACKAGE considered sufficient as an `.src.rpm` marker[1][3], some ancient versions of createrepo[4] contain a disappointing typo[2] that makes yum-powered package distribution impossible.
So we also fill up TAG_SOURCERPM to make it possible to bypass such a bug using just rpmprops entry like `SourceRPM: "dummy.src.rpm"`.

[1] http://yum.baseurl.org/gitweb?p=yum.git;a=commitdiff;h=bacabf7ad9883d191fad9c9ffdee4b7c3b2024df
[2] http://createrepo.baseurl.org/gitweb?p=createrepo.git;a=commitdiff;h=20a4999107bd0493dbd3994d23a60b43a601b1dd
[3] http://www.rpm.org/max-rpm-snapshot/s1-rpm-file-format-rpm-file-format.html#S3-RPM-FILE-FORMAT-HEADER-TAG-LISTING
[4] e.g. ones that shipped as part of CentOS 5 distribution (0.4.11-3.el5 as of today)