Fix PopulateUpstreamToDrake for checkout not named drake

[jwnimmer-tri]

The prior version relied on "/drake/" appearing in absolute pathnames to URDF files, then looked for package.xml files in the trip from the URDF down to drake's root. This worked when all resources were in drake-distro/drake. When we got rid of drake-distro, it still worked if your git clone was named drake, or if you were running a test in the bazel sandbox where the workspace is named drake. If you were running from the command line, it would give up because it didn't know where Drake was.

The fix is to use GetDrakePath to establish the coordinate to search up until. This still might fail if the URDF is from some foreign Drake tree, but GetDrakePath is finding the current tree (it won't add the foreign packages), but at least this is an improvement.

Fixes #7855. Relates #6996.

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This change is 

[jwnimmer-tri]

+@soonho-tri for all review please. I will find someone to test locally also.

[soonho-tri]

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Reviewed 2 of 2 files at r1.
Review status: all files reviewed at latest revision, 3 unresolved discussions.

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multibody/parsers/package_map.cc, line 135 at r1 (raw file):

void PackageMap::PopulateUpstreamToDrakeHelper(
    const string& directory,
    const std::string& stop_at_directory) {

BTW, std:: can be removed.

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multibody/parsers/package_map.cc, line 139 at r1 (raw file):

  // If we've reached the top, then stop searching.
  if (directory.length() <= stop_at_directory.length()) {

BTW, this only works if we know that stop_at_directory is an ancestor of directory. Do we have this as a pre-condition?

OK.

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multibody/parsers/package_map.h, line 82 at r1 (raw file):

  // Recursively searches up the directory path searching for package.xml files.
  // Adds the packages defined by these package.xml files to this PackageMap.
  // This method is intended to be called by PopulateUpstreamToDrake().

BTW, need to update the doc section?

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Comments from Reviewable

[soonho-tri]

Reviewed 2 of 2 files at r2.
Review status: all files reviewed at latest revision, all discussions resolved, all commit checks successful.

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Comments from Reviewable

[soonho-tri]

I will find someone to test locally also.

Good to merge this as @rdeits tested it?

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Review status: all files reviewed at latest revision, all discussions resolved, all commit checks successful.

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Comments from Reviewable

[jwnimmer-tri]

Hmm, this had the side-effect that merely loading URDFs requires that FindResource works now, even if the user never wants or tries to use FindResource to locate URDFs. I will try to follow-up and weaken that.