CRM_Extension_Browser - Allow different cache files for different feeds

[totten]

The motivation here is to support these two different CLI commands:

cv dl myextension
cv dl myextension --dev

The two commands differ in that they pull from different feeds -- e.g. the
stable feed https://civicrm.org/extdir/ver={ver}|cms={uf} vs the
developmental feed https://civicrm.org/extdir/ver={ver}|cms={uf}|status=.
However, this creates the real possibility that the user might go back/forth
among different feeds (omitting/enabling the --dev option per whim).

[totten]

Updated and squashed patch.

Jenkins reported a failure reading the newly named test file. This is odd because:

1. The failure makes sense with old code and new code.
2. The failure doesn't reproduce locally (even if I clear out cache files before running it).

Theory: the code was buggy before, but it was obscured because (a) error-reporting is not consistent across PHP configs and (b) something in the test-suite (preceding test) has a side-effect which was producing that file. (Specifically, api_v3_ExtensionTest in 4.7 plays around with ext_repo_url.)

We'll see if the revised patch (with proper guard) fixes it.

[colemanw]

Makes sense to me.