Capture libeigen's header correctly

libeigen exports entire directory, but only two directories contain
actual exported headers. And some of headers have no extension. This
adds a special behaviour for external/eigen when capturing snapshot.

Exempt-From-Owner-Approval: cherry-pick from internal branch

Bug: 157106227
Test: m vendor-snapshot captures libeigen's headers
Change-Id: I82f27d4c6c4d472f47d54c545328150697b8cda3
Merged-In: I82f27d4c6c4d472f47d54c545328150697b8cda3
(cherry picked from commit 67f3dbca)