Make sure .babelrc is a file, not a directory

[privatenumber]

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In particular the portion on Commit Message Formatting

Please check if the PR fulfills these requirements

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

What is the current behavior? (You can also link to an open issue here)
If there is a directory called .babelrc in the repo root or it's parent, the build breaks with an error: Module build failed: Error: EISDIR: illegal operation on a directory, read stemming from find (/Users/hiroki/Documents/Gits/babel-loader/lib/resolve-rc.js:21:12)

What is the new behavior?
If the detected .babelrc is not a file, it ignores it and moves on.

Does this PR introduce a breaking change?

• Yes
• No

If this PR contains a breaking change, please describe the following...

• Impact:
• Migration path for existing applications:
• Github Issue(s) this is regarding:

Other information:
There is logic in babel-core in build-config-chain.js that checks if .babelrc exists but neglects to check if it's a file. I will make a PR fixing it there as well.

[codecov]

Codecov Report

Merging #427 into 7.0 will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##              7.0     #427   +/-   ##
=======================================
  Coverage   83.23%   83.23%           
=======================================
  Files           6        6           
  Lines         173      173           
  Branches       41       41           
=======================================
  Hits          144      144           
  Misses         12       12           
  Partials       17       17

Impacted Files	Coverage Δ
src/utils/exists.js	100% <100%> (ø)	⬆️

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[loganfsmyth]

We can certainly throw a better error, but I don't think silently ignoring it makes sense?

[privatenumber]

If there's a directory that is coincidentally named .babelrc in it's parent, that has nothing to do with the build (or perhaps not even Babel), does it make sense to throw an error that will prevent the build from happening?

That is the case I am currently encountering. When executing Babel on a shared server I do not have admin control in, it fails because someone created a .babelrc folder.

I think the particular part of the code I updated is not only concerned with whether .babelrc exists, but more so that it's readable.

[hiroppy]

babel/babel#5535

[loganfsmyth]

Hmm, that's a fair point, if a little gross. I guess I can see the utility of this.