Don't fetch recursive dependencies from files with +build ignore tag

[alexedwards]

Currently gvt fetches recursive dependencies that are listed in files which have the // +build ignore tag. This seems to go against the expectation (as mentioned in the the README) that only files relevant to the Go compiler are fetched.

A real-life example of where this is a problem is when vendoring github.com/mattn/go-sqlite3. This package contains a tool directory with some helpers which don't form part of the core package. All files in the tool directory use the // +build ignore tag.

$ gvt fetch github.com/mattn/go-sqlite3
2016/12/08 16:00:03 Fetching: github.com/mattn/go-sqlite3
2016/12/08 16:00:10 · Fetching recursive dependency: golang.org/x/net/context
2016/12/08 16:00:14 · Fetching recursive dependency: github.com/PuerkitoBio/goquery
2016/12/08 16:00:21 ·· Fetching recursive dependency: golang.org/x/net/html
2016/12/08 16:00:21 ··· Fetching recursive dependency: golang.org/x/text/encoding/htmlindex
2016/12/08 16:00:40 ···· Fetching recursive dependency: golang.org/x/text/encoding/charmap
2016/12/08 16:00:40 ····· Fetching recursive dependency: golang.org/x/text/encoding
2016/12/08 16:00:40 ······ Fetching recursive dependency: golang.org/x/text/transform
2016/12/08 16:00:40 ······ Fetching recursive dependency: golang.org/x/text/runes
2016/12/08 16:00:40 ······ Fetching recursive dependency: golang.org/x/text/internal/utf8internal
2016/12/08 16:00:40 ······ Fetching recursive dependency: golang.org/x/text/language
2016/12/08 16:00:40 ······· Fetching recursive dependency: golang.org/x/text/internal/gen
2016/12/08 16:00:40 ········ Fetching recursive dependency: golang.org/x/text/unicode/cldr
2016/12/08 16:00:40 ······· Fetching recursive dependency: golang.org/x/text/internal/tag
2016/12/08 16:00:40 ·· Fetching recursive dependency: github.com/andybalholm/cascadia

Because gvt doesn't ignore the files in the tool directory, github.com/PuerkitoBio/goquery (and subsequently, all it's dependencies) get vendored, even though they will never be used in the compiled application.

[alexedwards]

With go mod due to be shipping in 1.11 as the official tool for managing versions I'm not sure that fixing this would be a good use of anyone's time. Closing.