rustpkg should make locally cached files read-only

[catamorphism]

Users could easily lose track of which source files are locally-cached copies of a dependency specified by a URL-like package ID. When rustpkg fetches source from a remote git repo, it should make the local copies read-only. (This won't change anyone from changing permissions on the file, but at least it will force them to think about what they're doing.)

As motivation for this, @metajack pointed out http://forums.thedailywtf.com/forums/t/27755.aspx

[catamorphism]

nominating for milestone 3, feature-complete

[graydon]

accepted for feature-complete milestone

[thestinger]

Bumping for triage, still unimplemented.

[catamorphism]

#9732 will close this.