BUG: ExternalData: Collapse ../ components in DATA{} paths

Relative path components need to be normalized out even if they appear
in the middle of a caller-supplied string.

Change-Id: I849b8af682053a5fa4a4e604247db2d90f8126c7
Merge topic 'generalize-ExternalData'

31752a3 BUG: ExternalData: Collapse ../ components in DATA{} paths