Git import script can't pull multiple repos into the same directory

[mchurichi]

As it is right now, if we want to use the pull_external script to import files from different repositories into the same directory, it is not possible because the yaml we need to do it will have duplicated keys (spire-docs in the example). The last occurrence of the key will override all the previous occurrences.

For example, this yaml will only pull JWT-SVID.md:

spire-docs:
    source: https://github.com/spiffe/spire
    pullFiles:
        - doc/upgrading.md
spire-docs:
    source: https://github.com/spiffe/spiffe
    pullFiles:
        - standards/JWT-SVID.md