repoman: tell user which mirror identifiers contain SRC_URI

before the patch:
    $ repoman full

    RepoMan scours the neighborhood...
      metadata.missing              1
       dev-haskell/qthaskellc/metadata.xml
      SRC_URI.mirror                1
       dev-haskell/qthaskellc/qthaskellc-1.1.4.ebuild: 'http://download.berlios.de/' found in thirdpartymirrors

after the patch:
    $ repoman full

    RepoMan scours the neighborhood...
      metadata.missing              1
       dev-haskell/qthaskellc/metadata.xml
      SRC_URI.mirror                1
       dev-haskell/qthaskellc/qthaskellc-1.1.4.ebuild: 'http://download.berlios.de/' found in thirdpartymirrors: 'mirror://berlios'

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>
repoman: improve thirdpartymirrors output