api should reject manual release targets pointing back to itself

[dirkmueller]

Issue Description

recently I accidentally misconfired a prj in the following way:

How to Reproduce

<project name="home:dirkmueller:foo">
  <repository name="containerfile">
    <releasetarget project="home:dirkmueller:foo" repository="images" trigger="manual"/>
    <path project="SUSE:SLE-15-SP6:GA" repository="standard"/>
    <arch>x86_64</arch>
    <arch>aarch64</arch>
  </repository>
  <repository name="images">
    <releasetarget project="home:dirkmueller:foo" repository="images" trigger="manual"/>
    <path project="SUSE:SLE-15-SP6:GA" repository="standard"/>
    <arch>x86_64</arch>
    <arch>aarch64</arch>
  </repository>

I mistyped the "project" in here, it should have been "home:dirkmueller:foo:ToTest". when I triggered a release, the build service basically became unresponsive :

outdated (was: succeeded: Releasing via trigger event, Releasing via trigger event, Releasing via trigger event, Releasing via trigger event, Releasing via trigger event, Releasing via trigger event, Releasing via trigger event, Releasing via trigger event, Releasing via trigger event, Releasing via trigger event, Releasing via trigger event, Releasing via trigger event, Releasing via trigger event, Releasing via trigger event, Releasing via trigger event, Releasing via trigger event, Releasing via trigger event, Releasing via trigger event, Releasing via trigger event)

[hennevogel]

As soon as you invent links you need to take care of cycles. In the case of releasetarget no one did so far...