Code Review: build-server parallel checkouts changes

[mattpen]

@chrisklus is going to describe what needs to be reviewed and assign this to @jonathanolson when ready.

[chrisklus]

This issue is for reviewing work done in phetsims/chipper#1112 and #330.

Please review the following commits in the order below:

3e1ad11
6fb1ab8
1309fd2
ed5f725
a911b3f
aca3e0e
4f25e7a
a3510b9
5cfb8d1
0a9f16e
ed136dd
a7796f3

The goal of these changes was to remove relative links to where the master checkouts formerly lived and to likewise remove any incorrect dependencies on the master checkouts during builds.

To have confidence that things are working correctly, we restructured the sub-directories of /data/share/phet/build-server on phet-server2 to contain image-repos (where the master checkouts now live, used only to get sim images), perennial, and release-branches. This means that anything referencing a master checkout in the original location will fail.

Successful tests of these changes are outlined in #330 (comment).

It looks like phetsims/chipper#1112 still needs to be wrapped up - either with a review of a3257f4, or a better solution as mentioned in phetsims/chipper#1112 (comment).

Please let me know if you'd like to do any of this review together or need more explanation for any parts.