"path must be relative" in filesets is holdover convention from debian packaging, causes grief

[Dan Kegel]

Hi!

When generating snapcraft.yaml from e.g. CMakeLists.txt for a large project,
I wanted to exclude some dev files which live at a
configuration-dependent location, e.g.

    filesets:
      dev:
        - - at FOO@/include
        - - at FOO@/lib/*.a
        - -usr/share/doc

Can you spot the usability problem?  This fails with the error

path "/opt/foo/include" must be relative

Please, for the love of $deity, don't make the same usability mistake
debian packaging did.
Don't make authors add special cases in their script to strip off the
leading slash of paths.

That is all.  Thank you.

:-)

- Dan