show_news(): Support NEWS in Rd and md formats and under inst

[arcresu]

Fixes #2499.

Exactly mirrors the search path documented in utils::news() for NEWS files rather than using the regex approach proposed in the issue.

The existing code could only handle plain NEWS files. In order to support the alternative formats, I had to mirror what utils::news was doing via internal tools machinery in tools:::.build_news_db().

I added some tests partly to convince myself it was working. Perhaps you don't want to take all of these though since it's effectively exercising the internal functions in tools more than devtools per se.

[arcresu]

I discovered that the NEWS-parsing machinery in tools skips entries were the version string is malformed according to .standard_regexps()$valid_package_version. In particular, the de facto standard first entry of packagename (development version) is skipped because "(development version)" doesn't match that regex. This is unfortunate, since it's probably exactly the entry you'd want to look at.

If there are no valid entries, the news_db builder functions return NULL instead of a news database, and forwarding that on to utils::news(db = NULL) causes it to display the NEWS for R itself rather than the package. This was the cause of the earlier CI failures in this PR.

In combination, it means that in a new package with a NEWS.md generated by usethis::use_news_md(), if you call devtools::show_news(), you will see an error Unable to parse NEWS, since there are no entries with valid version strings. All in all, this PR still improves the state of affairs by supporting all formats and locations of NEWS that R supports.

[hadley]

Thanks @acresu! I pulled out the finding into a separate function so we could test that, without being quite so vulnerable to changes in other news machinery in base R.