Add example of PackageMap and an ament resource

[EricCousineau-TRI]

Condensed, but less functional, version of #1

FYI @adeeb10abbas @jwnimmer-tri This example won't pass on CI or w/ Bazel archive, but should work if you use a package resource that actually exists. In my case, I dumpster dove to find the test URDF. Better solution is to vendor our own.

Relates ament changes in #133

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