feat: absolute imports (#63)

* feat: go-like import style

jb now creates a directory structure inside of vendor/ that is similar to how go
does (github.com/grafana/jsonnet-libs). This is reflected in the final import
paths, which means they will be go-like

* refactor(spec/deps): named regexs

* feat: make goImportStyle configurable

Defaults to off, can be enabled in `jsonnetfile.json`

* fix: integration test

* doc: license headers

* fix(deps): remove GO_IMPORT_STYLE

not an option anymore, will always do so and symlink

* feat: symlink to legacy location

* feat: allow to disable legacy links

* fix(test): legacyImports in integration tests

* fix(spec): test

* fix: respect legacyName aliases

It was possible to alias packages by changing `name` previously.

While names are now absolute (and computed), legacy links should still respect
old aliases to avoid breaking code.

* fix(test): integration

* fix(init): keep legacyImports enabled for now

* feat: rewrite imports

adds a command to automatically rewrite imports from legacy to absolute style

* fix(tool): rewrite confused by prefixing packages

When a package was a prefix of another one, it broke.
Fixed that by using a proper regular expression. Added a test to make sure it
works as expected

* Update cmd/jb/init.go

* fix: exclude local packages from legacy linking

They actually still use the old style, which is fine. LegacyLinking
messed them up, but from now on it just ignores symlinks that match a localPackage.

README.md

@@ -106,6 +106,9 @@ Commands:
   update
     Update all dependencies.
 
+  rewrite
+    Automatically rewrite legacy imports to absolute ones
+
 
 ```