Fix #218. Set container_source to source for references

[rly]

Fix #218. When handling references, calls to BuildManager.build now include a source parameter. container_source is then set appropriately on line 159.

[codecov]

Codecov Report

Merging #219 into dev will not change coverage.
The diff coverage is 12.5%.

@@           Coverage Diff           @@
##              dev     #219   +/-   ##
=======================================
  Coverage   70.38%   70.38%           
=======================================
  Files          31       31           
  Lines        5920     5920           
  Branches     1389     1389           
=======================================
  Hits         4167     4167           
  Misses       1322     1322           
  Partials      431      431

Impacted Files	Coverage Δ
src/hdmf/build/map.py	66.96% <12.5%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 448b467...72a8433. Read the comment docs.