When populating ExportMap, only load file if it's not ignored

[brendo]

I was debugging a project trying to understand better how the import/no-deprecated rule works. While doing this I discovered that ExportMap.for will do a I/O hit to read the path even if the path is ignored:

https://github.com/benmosher/eslint-plugin-import/blob/112a0bf442e52b25cd7029a9905df2508e191ac1/src/ExportMap.js#L313-L320

When the path is ignored, the function early exits and the content variable is not referenced, so it's essentially a wasted I/O hit.

This PR tweaks the condition to do the ignore check in one block, and check ambiguity in another.

In theory this should result in a performance boost for projects that ignore many paths as they will be skipped based off the ignore, rather than being loaded from the file system. It also cleans up the log a little bit because we know exactly why the path was ignored, because of the regex, or because it's ambiguous.

[coveralls]

Coverage decreased (-0.2%) to 96.115% when pulling fb8ae71 on brendo:fix_only-load-file-when-needed into 1b96580 on benmosher:master.

[coveralls]

Coverage decreased (-0.3%) to 96.172% when pulling fa64e29 on brendo:fix_only-load-file-when-needed into 112a0bf on benmosher:master.

[coveralls]

Coverage decreased (-0.3%) to 96.172% when pulling fa64e29 on brendo:fix_only-load-file-when-needed into 112a0bf on benmosher:master.

[coveralls]

Coverage decreased (-0.3%) to 96.172% when pulling fa64e29 on brendo:fix_only-load-file-when-needed into 112a0bf on benmosher:master.

[coveralls]

Coverage decreased (-0.3%) to 96.172% when pulling fa64e29 on brendo:fix_only-load-file-when-needed into 112a0bf on benmosher:master.

[coveralls]

Coverage decreased (-0.3%) to 96.172% when pulling fa64e29 on brendo:fix_only-load-file-when-needed into 112a0bf on benmosher:master.

[coveralls]

Coverage decreased (-0.3%) to 96.172% when pulling fa64e29 on brendo:fix_only-load-file-when-needed into 112a0bf on benmosher:master.